Longitudinal investigation of psychological outcomes associated with screen use in Danish preschool children: study protocol for The Digital Child

Abstract

Introduction

Despite an increasing use of screens among preschool children and evidence suggesting potential adverse effects, there is a paucity of longitudinal research that aims to disentangle the multifaceted components of screen use and their unique associations with development. We present a protocol for a large-scale national longitudinal study with repeated measurements in Danish preschool children, with the aim of investigating the cross-sectional and cross-lagged longitudinal associations between screen use and psychological outcomes.

Methods and analysis

The Digital Child Study is a national prospective observational cohort of Danish preschool children. Baseline parent-report data collection commenced in 2024 via online questionnaires, and in total will include three time points over 1 year: baseline (age 4 years), and follow-ups at 6 and 12 months (ages 4.5 and 5 years). Participants were divided into two waves based on birth dates, starting in March and September 2024. Recruitment targeted parents and primary caregivers of all Danish children born between specific dates in 2020. Of 30 235 children whose parents were sent invitations, baseline questionnaire data were available for 11 690 (39%).

Children’s screen use was measured by detailed information of amount, content and timing of children’s screen use, and the broader context, incorporating parental mediation strategies, attitudes, motivations and practices. Cognitive and socioemotional developmental outcomes were measured using validated tools such as the Strengths and Difficulties Questionnaire, the Nordic Five-to-Fifteen parent questionnaire and the Behaviour Rating Inventory of Executive Function—Preschool Version. Questionnaire data will be linked to national social and health registries to enable long-term follow-up. Statistical analyses will include longitudinal modelling to explore associations between screen use and developmental outcomes, with sensitivity analyses for robustness. The study’s large sample size provides high statistical power to detect meaningful effects.

Ethics and dissemination

The study adheres to ethical research guidelines, ensuring voluntary participation, confidentiality and compliance with data protection laws, with approvals from relevant authorities. Findings will be disseminated through peer-reviewed publications, conferences and plain-language summaries to engage stakeholders and the broader community.

STRENGTHS AND LIMITATIONS OF THIS STUDY.

• The Digital Child Study invited all parents and primary caregivers in Denmark with children born within 6 specified months.

• Parents of more than 11 000 4-year-old children completed measures of screen use at three different time points over a 1-year period.

• The design moves beyond the typical focus on screen time, encompassing screen content and timing, but also the broader context of the family screen culture.

• The study employed well-validated and reliable measures of cognitive and socioemotional developmental outcomes.

• Parent-report data can be vulnerable to random or systematic misreporting, or socially desirable responding, which may lead to potential misclassification and bias of the associations.

Introduction

For the current generation of young children, handheld screen-based devices are a fixture of everyday life. The multifunctional nature of handheld screens has led to significantly increased use of overall screen-based technology for many families during the last decade, and today’s children spend notably more time on screens than previous generations.1,3 Additionally, children 5 years and younger are the fastest-growing users of screens,^{1 4 5} and only a minority of children meet the screen time guidelines set by the American Academy of Pediatrics,⁶ the WHO⁷ and others, of no more than 1 hour of daily screen time for ages 2–5 years.¹ Awareness of this data has fuelled both scientific and public debate surrounding the potential risks and benefits of screen use for young children’s psychological development.

There is evidence to suggest that screen use may have adverse effects on children’s psychological development,8,20 underscoring the need to better understand childhood screen use as a potential public health concern. Although there is evidence that some forms of screen use can be developmentally beneficial (eg, educational usage associated with improved language skills¹⁶ and touchscreen scrolling associated with fine motor skill development),²¹ there is a growing body of evidence to suggest that, overall, developmental outcomes are poorer, including language, communication, executive function and memory.812,14 Recent systematic reviews and meta-analyses further underscore these links, such as the link between prolonged screen time with changes in sleep patterns and diminished cognitive performance in preschool children.911 15,17 20 22 Studies that have investigated the socioemotional effects of screen use in young children have found increased screen use to be associated with poorer social skills, increased internalising and externalising problems, bullying behaviour and more emotional problems.^{8 9 12 13 15 19} Several systematic reviews have found that screen use was associated with behavioural problems, such as aggressive behaviour problems,^{13 19} and poorer psychosocial health, such as increased anxious or depressive symptoms and emotional symptoms.911,13 15 Although the majority of research in screen use and child development systematic reviews and meta-analyses are cross-sectional in nature (reflecting the predominant methodology in the literature), together these studies build a compelling argument of a meaningful association between screen use and child developmental outcomes.

Yet, the reviews highlight critical methodological limitations, including a reliance on cross-sectional data and narrow conceptualisations of screen use.911 13 15,17 To fully understand which components of screen use may be associated with children’s cognitive development, it is essential to consider factors such as context and content of use, solo versus co-use, parental screen mediation and multiscreen use. Timing of screen use is also a factor, as evening screen use and bedroom access to devices are linked to poorer sleep among children,²³ increasing the risk of cognitive problems.⁹ In addition, a large multicohort study of over 384 591 participants from 55 countries found that as age of first use of digital devices increased, the likelihood of being a regular device user decreased at age 15 years.²⁴ Similarly, evidence shows that child ecological factors related specifically to parental decision-making, such as age-inappropriate screen content, co-use and caregiver screen use, have different effects on cognitive and psychosocial outcomes among children.¹¹ Parental behaviour, especially parental screen use, can significantly impact parent-child interactions, which in turn may impact children’s developmental outcomes.^{22 25 26} These findings suggest that the family environment is a critical factor in understanding the associations between screen use and child development. The interruption of parent-child interactions by technology has also been associated with problematic child screen use.²⁷ Given a young child’s limited autonomy, incorporating parental perceptions, practices and challenges in future research is of primary importance. However, there is a lack of longitudinal studies exploring the potential effects of family factors on child screen use in general, and important moderating factors, such as household features, parental involvement and socioeconomic status specifically.^{12 22}

Furthermore, there is a dearth of research concerned with newer forms of screen-based devices (ie, beyond television),^{13 15 28} as well as the ways that children use modern technologies. Compared with television viewing, the use of handheld touch screen devices differs in terms of both user interactivity and the nature of the content available. Modern screen-based devices often present highly curated and targeted content to young children. However, there is limited understanding of any potentially new or different mechanisms (compared with television) that may underpin associations between usage and psychological or socioemotional development. One reason that mechanisms are largely unexplored is a lack of robust longitudinal studies that can reliably explore potentially causal factors beyond basic measurements of time spent on screens. Although time is a useful metric, there are arguably equally or more important aspects of screen use that could help explain known associations with child development. For example, parental motivations for provision of touchscreen devices (eg, emotion regulation of toddlers with socioemotional delays),²⁹ or parental beliefs around ‘healthy’ use of screens (eg, educative content vs entertainment). There is evidence that children who are experiencing negative psychological symptoms or a dysfunctional parent-child relationship are also more likely to develop a problematic screen use,³⁰ potentially due to an attempt to manage their own emotional distress by escaping into the virtual world.

While there are several longitudinal studies that are informative, few have been specifically designed to undertake a holistic examination of child screen use during the early years. For example, a longitudinal study of Australian children found that various child, parenting, home and neighbourhood variables at age 4–5 years predicted daily screen time at age 6–7 years.³¹ This study provides strong support for the idea that early childhood screen and family factors can predict later screen behaviour, but as the data were a subset of a much larger developmental panel study (Longitudinal Study of Australian Children), the outcome measure was limited to daily screen use only. Another recent longitudinal study examined cognitive development outcomes of US adolescents at ages 11–12 years after a 2-year follow-up.³² The authors assessed both types of screen use (eg, texting, video watching, gaming) and screen time to predict several outcomes including depressive symptoms, attention-deficit/hyperactivity and oppositional defiance symptoms. Both types of screen use and screen time predicted poorer outcomes at Time 2, with some racial nuances reported. However, again, although very informative, the study included a limited number of ecological and contextual variables.

Although the scientific literature on screen use is growing, the multifaceted nature of screen use has been largely overlooked in favour of a simplistic unidimensional measure of overall screen time when evaluating the benefits and risks in early childhood. Thus, as most previous studies have focused on overall screen time, there is currently a lack of data considering the differential aspects of screen use that include content, type of screen, co-use, situations, purpose and context. The implications are that many studies draw conclusions about the effects of screen use by relying only (or mostly) on a measure of screen time. This approach has been useful in the early stages of early childhood screen use research; however, it is now clear that time is only one aspect of a complex ecology involving separate but related factors that likely act in concert to affect development. Additionally, the lack of longitudinal studies addressing a variety of factors in a child’s ecosystem limits the ability to rule out causality. Therefore, we have launched a longitudinal cohort study, The Digital Child Study, that invited a national sample of parents and caregivers of Danish preschool children. The Digital Child Study takes a multifaceted view of screen use, including the amount, content and timing of use, and the broader ecological context surrounding the child’s screen use.

Objectives

The main objective is to investigate the cross-sectional and cross-lagged longitudinal associations between screen use and psychological outcomes among 4–5 year-old Danish children. Overall, the study will investigate:

1. The prevalence and variability in screen use among 4–5 year-old children in Denmark.

2. If the amount, content, timing and context of screen use are associated with cognitive development (attention, working memory, language and executive function) or socioemotional development (peer relations, emotion regulation, hyperactivity and behavioural problems).

3. The potential cross-sectional and longitudinal mechanisms that can explain psychological consequences of screen use.

Methods and analysis

Study design

The Digital Child Study is a national prospective observational study examining Danish preschool children’s screen use and related psychological outcomes. The study is carried out by the Danish National Institute of Public Health (NIPH), University of Southern Denmark.

Participants were followed for 1 year, with data collection relying on parent-reported online questionnaires administered at three different assessments: baseline at age 4 years, a shortened version at 6 months and then full reassessment at 12 months. The corresponding child ages were 4, 4.5 and 5 years, respectively. Every Danish parent or caregiver of a 4-year-old child who met the inclusion criteria was invited to participate in the study. The children were divided into two waves based on their birthdates, with wave 1 starting in March 2024 and wave 2 in September 2024. Each wave followed the same measurement schedule but was offset by 6 months, thus the data collections for both waves proceeded in parallel. The staggered design ensured that a large number of children within the same age span could be invited. In addition, the 6 months span ensured that weather and temperature in Denmark were approximately similar at the two time points.

An overview of the study design is provided in figure 1.

Figure 1. Study design of The Digital Child Study.

Recruitment

Parents were recruited using the Danish Civil Registration System. Danish residents are issued a unique Civil Personal Registration (CPR) number at birth or on immigration, and with permission from The Danish Health Data Authority, we used this unique CPR number to identify and define the study population. The population of interest was all children registered in Denmark born between specific dates in 2020, 1 February and 30 April (wave 1) or between 1 June and 31 August (wave 2), who met the following eligibility criteria: the child must (1) have a valid CPR number, (2) be alive, (3) reside in Denmark (excluding Greenland) as of the extraction date and (4) not have a name or address protection or legal guardianship restrictions. In the case of twins or multiple births, a data manager at NIPH included one child per set, which was defined as the one whose first name came last alphabetically. Additionally, at least one parent must have a valid CPR number, be alive, be the legal custodian, cohabitate with the child and have no name or address protection or legal guardianship restrictions as of the extraction date.

Parents were invited to complete the questionnaires by an invitation letter distributed using a national and secure digital post system used for Government and private correspondence in Denmark (often referred to in Danish as ‘e-Boks’). The invitation letter included a Plain Language Statement and consent information, and a link to the questionnaire. The study information and baseline questionnaire were hosted in a secure online survey platform (‘SurveyXact by Ramboll’). The invitation letter provided a description of the survey, its purpose, procedure and information on how to access future research. The parents were informed that the survey allowed only one set of responses per child, allowing them to complete the questionnaire individually or together. Both parents shared the same respondent key to ensure that there was no duplicate data should both parents attempt the survey for the same child. Up to two reminders were sent to non-responders after approximately 8 and 11 days. To enhance participation, all invitation letters allowed parents to opt in to a lottery for a gift card, drawn at the conclusion of each data collection point. Across the study period, eight gift cards were drawn, valued at 3000–5000 DKK (approx. $470–$780).

Recruitment for The Digital Child Study began in March 2024, and we anticipate the cohort recruitment to be completed in October 2025. At the time of writing, the first data collection took place between 12 March and 8 April 2024 (wave 1 baseline data collection) and second data collection between 11 September and 11 October 2024 (wave 1 first follow-up and wave 2 baseline data collection). The third data collection took place between 12 March and 11 April 2025, and the last data collection will begin mid-September 2025. Weather conditions in Denmark, specifically sunshine hours and precipitation, were monitored and systematically recorded throughout all data collection periods to assess potential environmental effects.

Study population

A total of 55 730 parents and caregivers of eligible children were invited to enrol in the study on behalf of their child. This represented 30 235 Danish children. In total, parents of 11 749 children participated in the baseline data collection, corresponding to a response rate of 39%. We excluded children (n=59) with discrepancies in a minimum of two of the parent-reported birthday information (either day, month or year) compared with the register-based information, as we assumed the parent had mistakenly answered based on a sibling. This resulted in a baseline sample of 11 690 children.

For the first follow-up, we invited parents of children who initiated answering the questionnaire and completed at least the question of the child’s total daily screen time. This question was a primary variable of interest and we, thus, ensured that we had some foundational data on screen exposure for all re-invited respondents. For wave 1, we collected data from parents for a total of 2789 children for the first follow-up, corresponding to a response rate of 53%. For wave 2, we collected data from parents for a total of 3044 children, corresponding to a response rate of 49%.

For the second and final follow-up, we will invite all parents who were invited at baseline (n=55 730). Parents who did not respond to the baseline questionnaire will receive a full questionnaire including all background information. Although parents who respond for the first time at the second follow-up cannot be used in longitudinal analyses, they will still contribute valuable cross-sectional information on screen use and potentially be used in future follow-up studies. At the time of writing, we collected data from parents of 2869 for the second follow-up of wave 1, resulting in a response rate of 20%. Among parents who participated in the baseline questionnaire, the response rate was 35%, whereas it was 11% among those who did not respond initially. We anticipate collecting data from approximately 3200 children for the second follow-up for wave 2, assuming a similar response rate (≈20 %).

A detailed overview of the recruitment process is illustrated in figure 2.

Figure 2. The Digital Child Study flow chart.

Measures

Data collection consists of a series of parent-reported questionnaires. The questionnaires are constructed from a combination of previously used items and new items developed for the purpose. The baseline questionnaire (age 4 years) included background characteristics of the parents, child and home environment, various elements of parent and child screen use, and measures of child psychological development. The first 6-month follow-up (age 4.5 years) solely focused on screen use. The second follow-up (age 5 years) replicated all baseline measures except for non-modifiable information (eg, birth weight, gestational age). The questionnaires took an average of 25 min at baseline and second follow-up, and 10 min for the first follow-up.

We conducted face validation by presenting the questionnaires to 15 parents with children between the ages of 3 and 5 years to assess clarity, relevance, and alignment with the study objectives. Feedback from this panel informed minor revisions to improve content appropriateness and ensure conceptual coherence. All translations were carried out by members of the research team, which includes both native Danish and English speakers. The translated versions were subsequently tested as part of the face validation process.

An overview of the different survey questions is provided in table 1.

Table 1. Overview of survey questions.

Category		Question types	Baseline	1st Follow-up	2nd Follow-up*
Demographic information		Gender	■
		Age	■		■
		Height/weight	■		■
		Birth timing in pregnancy	■
		Residential region	■
		Parent’s age	■
		Parent’s ethnicity	■
		Parent’s country of origin	■
		Parents’ education	■
		Parents’ occupation	■
		Household income	■
Family structure		Relation to the child/family relation	■	■	■
		Living conditions	■	■	■
		Daycare information	■		■
		Siblings living at home	■		■
		Birth order	■
Home environment		Child’s number of books	■		■
		Parent’s reading aloud at home	■		■
		Trips to the store	■		■
		Playing games or doing puzzles	■		■
		Outdoor play in different seasons	■		■
		Participation in household chores	■		■
Sleep duration		Bed and wake up time (weekdays and weekends)	■		■
General health		Diagnoses, health issues or developmental disorders†	■	■	■
		General health compared with peers	■	■	■
Child’s screen use (amount, content and timing)		Total daily screen time (weekdays and weekends)	■	■	■
		Daily screen time by device (weekdays and weekends)	■	■	■
		Daily screen time by activity (weekdays and weekends)	■	■	■
		Educational content	■	■	■
		Timing of screen use (weekdays and weekends)	■	■	■
		Average daily screen time from child’s device	■	■	■
		Three most-used apps from child’s device	■	■	■
Family screen culture (context)		Parental rules	■	■	■
		Child’s age of first solo tablet or smartphone use	■	■	■
		Child’s ownership of a smartphone or tablet	■	■	■
		Interaction with adult during screen use	■	■	■
		Multiscreen use	■	■	■
		Parental perceptions of child’s screen use†	■	■	■
		Parental motivations to allow screen use	■	■	■
		Specific situations (for additional screen time)	■‡	■	■
		Parents’ knowledge of and adherence to national guidelines	■‡	■	■
		Parents’ daily screen time with child nearby (weekdays and weekends)	■		■
Cognitive development	Executive function	Working memory (BRIEF-P)	■		■
		Emotional control (BRIEF-P)	■		■
		Plan/organise (BRIEF-P)	■		■
	Language milestone	Comprehension of spoken language (FTF)	■		■
		Verbal communication (FTF)	■		■
		Language compared with peers	■‡		■
		Bilingual or multilingual	■‡	■§	■
Socioemotional development	Mental health problems	Emotional symptoms (SDQ)	■		■
		Conduct problems (SDQ)	■		■
		Hyperactivity/inattention (SDQ)	■		■
		Peer relationship problems (SDQ)	■		■
		Prosocial behaviour (SDQ)	■		■
		Temperament compared with peers	■		■
	Social competences	Prosocial/communication skills (SCS-P)	■		■
		Emotional regulation skills (SCS-P)	■		■

^*Those not participating in the baseline survey received the full baseline questionnaire at 2. follow-up.

^†Wording of the question(s) was adapted after the baseline data collection of wave 1.

^‡Only included in baseline data collection of wave 2.

^§Only included in wave 1, as the question was included in the baseline data collection for wave 2.

BRIEF-P, the Behaviour Rating Inventory of Executive Function—Preschool Version; FTF, the Nordic Five-to-Fifteen parent questionnaire; SCS-P, the Social Competence Scale—Parent Version; SDQ, the Strengths and Difficulties Questionnaire.

Demographic information and family structure

Parents completed items about their demographic characteristics, including age, ethnicity and country of birth, along with socioeconomic questions regarding residential region, education, occupation and household income (full questions in online supplemental file 1 p. 1–3). The questions on parental age, education and occupation were inspired by questions from the Lolland-Falster Health cohort study.³³ In addition, parents provided information about family structure, including their relation to the child (ie, which parent was responding), living conditions, number of people in the household, number of siblings living at home and the child’s birth order. Additionally, they specified where their child is cared for during the day and the age at which their child began daycare or nursery.

Home environment

Home environment was assessed using a combination of three questions inspired by the Home Observation and Measurement of the Environment—Short Form (HOME-SF) scales,³⁴ along with four single measure questions (full questions in online supplemental information p. 4). The HOME-SF scale is an adaptation of the HOME inventory developed by Bradley and Caldwell.³⁴ In this study, the questions on home environment include one question assessing number of children’s books in paper form the child has, and questions assessing the frequency of parents’ reading aloud at home, trips to the supermarket or a shop, playing games or doing puzzles, outdoor play in different seasons (spring/summer and autumn/winter) and participation in household chores. For the frequency questions, five response options ranged from ‘Never’ to ‘Every day or almost every day’ with an additional option for ‘Don’t know’.

Sleep duration

Sleep duration was evaluated with two questions on hours of sleep per night from the Lolland-Falster Health cohort study³³ (4–10 years). Parents were asked to estimate when their child typically wakes up (‘When does your child typically wake up in the morning (or get woken up)?’) and falls asleep (‘When does your child typically fall asleep?’) with specific time intervals for both weekdays and weekends. Based on the questions, average sleep duration on weekday and weekend will be calculated.

General health

The physical health and well-being of the child was evaluated using two key questions. The first question was ‘Does your child have one or more diagnoses, health problems or developmental disorders?’. Response options were ‘Yes, please specify which one(s)’, ‘Unsure, please specify which one(s)’ and ‘No’. In the baseline questionnaire for wave 1, we included a different question from the Lolland-Falster Health cohort study³³ which we adapted after the first data collection. The question was ‘Does your child have any long-term illness, long-term effects of an injury, disability or other long-term condition? ‘Long-term refers to a duration of at least 3 months.’ with response options ‘Yes’ and ‘No’, including an option to specify which one(s) in an open text field.

The second key question was ‘How is your child’s general health compared with that of their peers?’ with the three following response options: ‘Better’, ‘Comparable to peers’ and ‘Worse’.

Child’s screen use

The primary exposure of interest is the children’s screen use, which is covered by questions constructed for this study and tested in pilot studies. Some questions are inspired by the SCREENS questionnaire (SCREENS-Q),³⁵ which is a Danish questionnaire designed to assess 6–10 year-old children’s screen use and its correlates. The questionnaire was developed through an iterative process involving experts and parents to ensure face and content validity and has demonstrated good internal consistency and moderate to substantial test-retest reliability. In this study, the questions cover detailed information on the amount, content and timing of a child’s screen use. All questions regarding child’s screen use are available in online supplemental information, where the exact wording and response options are provided, along with further details on the questions’ sources and any modifications made from SCREENS-Q. The following section offers a brief overview of these measures.

Overall screen time was measured in two different ways. First, parents were asked to report their child’s total daily screen time across all devices. In addition, parents were asked to provide more detailed information regarding their children’s daily screen time by device, on each of five devices (smartphone, tablet, computer, TV, game console). All frequency questions on screen time (total, by device and by activity) had the same response 14 categories spanning from ‘None’ to ‘More than 7 hours’. Parents were asked to delineate screen time separately for both weekdays and weekends.

Content was measured as daily screen time by activity, encompassing the following eight activities: YouTube, TikTok, media content (such as movies, cartoons and series), physically interactive games, sedentary games, video recording, video calls and other screen-related pursuits, separately for both weekdays and weekends. In addition, educational content was measured by asking parents how much of the child’s screen time they estimated was spent on activities primarily focused on learning (eg, letters, numbers, language and knowledge) with five response options ranging from ‘Very little of the time (around 0–20%)’ to ‘Almost all the time (around 81–100%)’.

Timing of screen use was measured by asking parents to identify when their child uses screen devices, with options including ‘within the first 30 min after waking up’, ‘while eating dinner’, ‘after daycare’, etc. Parents were able to select multiple responses of when their child typically uses screens, separately for weekdays and weekends.

Objective measures on screen time and content were measured if the child had their own smartphone or tablet. Parents were thus asked to use the settings on the device to objectively report the average daily screen time from the child’s device over the past week on that device, including the child’s three most-used apps. The questionnaire included instructions for both iOS (using the option of ‘Screen Time’ in settings) and Android (using ‘Digital Well-being’ or other monitoring tools, depending on the device), along with a link for a video tutorial showing how to retrieve screen time data for individual devices.

Family screen culture

Another primary exposure of interest is the broader screen culture within the family; the context of the child’s screen use. The questions aim to capture the family environment in which child screen use occurs, including parental mediation strategies, attitudes, motivations and practices. Screen habits and culture within the family household are covered by a combination of questions from the SCREENS-Q³⁵ and questions developed specifically for this study. All questions regarding family screen culture are fully available in online supplemental information, and the following section offers a brief overview of these measures.

Several dimensions were included to cover parental mediation strategies, assessing whether parents implement practices to guide or regulate their child’s screen use. Parental rules were measured by five statements from the SCREENS-Q³⁵ about common screen use mediation approaches and an additional two statements constructed for this study. Statements included whether parents set fixed screen time limits, establish specific times during the day when screens are allowed, or create rules about the types of content their child can access. For example, ‘The child must always ask for permission before using screen devices’ and ‘There are set limits on how much time the child may use screen devices’ with five options ranging from ‘Strongly agree’ to ‘Strongly disagree’. Response options were modified compared with the original scale that included two options. In addition, questions on child’s age of first solo tablet or smartphone use and child’s ownership of a smartphone or tablet were also included.

Interaction with adults during screen use was measured by asking about how much of the child’s screen time is typically spent in active interaction with an engaged adult, and multiscreen use was measured by asking parents how often their child uses more than one screen device at the same time. The two questions had five response options ranging from ‘Very little of the time (around 0–20%)’ to ‘Almost all the time (around 81–100%)’.

Parental perceptions of child’s screen use were assessed by a set of questions from the SCREENS-Q.³⁵ We included six statements from the original 16 items, addressing aspects such as the child’s preference for screen activities, the role of screens in soothing the child, and whether screen use fosters enjoyable conversations or enhances creativity. Statements included ‘Using screen devices often helps my child to calm down’ and ‘My child’s use of screen devices leads to many enjoyable conversations between us’ with five response options ranging from ‘Strongly agree’ to ‘Strongly disagree’. Response options were modified compared with the original scale that included four options.

Parental motivations to allow screen use included soothing an upset child, rewarding good behaviour or creating a quiet environment to be able to do adult tasks such as cooking. Statements included for example, ‘Think about when your child is upset or frustrated: How much of that time do you allow your child to use screen devices to calm them down?’ with five response options ranging from ‘Very little of the time (around 0–20%)’ to ‘Almost all the time (around 81–100%)’. Specific situations where parents may allow their child additional screen time included scenarios such as when the child is sick, during transport, on holidays, when the child is cared for by others or when the child is with the other parent (in cases of separate households). Parents could select multiple responses and were able to specify other situations in an open text field.

Parents’ knowledge of and adherence to national guidelines was included in the study in response to newly formulated screen use guidelines released by the Danish Health Authority on 12 June 2024, recommending a maximum of 1 hour of daily screen time for children aged 2–4 years. With the release, we took the opportunity to add two additional questions to the baseline survey for wave 2 and all subsequent follow-up data collections as a means to assess the effectiveness of the introduction of guidelines. The first question assessed the knowledge: ‘Are you familiar with the Danish Health Authority’s new recommendations on daily screen time for 2–4 year-olds published in June 2024?’. The second question assessed adherence: ‘Do you try to follow these recommendations when it comes to your child?’.

Parents’ daily screen time with child nearby is covered by single questions inspired by those in the SCREENS questionnaire.³⁵ Parents were asked: ‘In the past month, how much time have you typically spent per day on screen devices at home while your child is nearby (both screen use for work and leisure)?’, separately for weekdays and weekends. Two additional questions assessed screen use of the other parent and cohabiting partner, if relevant. Response options spanned from ‘None’ to ‘More than 7 hours’.

Executive function

The Behaviour Rating Inventory of Executive Function, Preschool Version (BRIEF-P)^{36 37} is widely used standardised rating scale designed to assess the range of executive function in preschool children aged 2–5 years. Several studies have demonstrated its psychometric properties and validity as a screening measure of executive function difficulties in early childhood across different populations.^{38 39} The questionnaire is designed to be completed by parents or other caregivers with 63 items in five non-overlapping and validated scales.^{37 40} In this study, we used three subscales with 37 items in total: ‘Working memory’ (17 items), ‘Emotional control’ (10 items) and ‘Plan/organise’ (10 items). Parents were asked to rate their child’s behaviour in terms of how often, in the last 6 months, the behaviour has been a problem. Each statement is rated on a 3-point Likert scale: ‘Never (1), ‘Sometimes’ (2) and ‘Often’ (3). Total scores for each subscale can be calculated and subscales with 1–2 missing items are replaced with a score of 1.

Language milestones

The Nordic Five-to-Fifteen parent questionnaire (FTF)⁴¹ assesses the skills and behaviours of children in various domains of development. The FTF is a validated questionnaire for children developed by a Nordic expert group and is widely used in Nordic countries to detect atypical development in children.⁴¹ The FTF has been found to be a valid screening instrument among 5-year-old children⁴² and has been used in several Danish settings.^{43 44} To assess language milestones, we used nine questions from two subdomains: ‘Comprehension of spoken language’ and ‘Verbal communication’. Parents answer each statement by comparing their child to their peers with a 3-point Likert response scale: ‘Definitely applies’ (2), ‘Applies sometimes or to some extent’ (1) and ‘Does not apply’ (0). A mean score for each domain can be calculated with a lower score indicating a child is achieving language milestones. For verbal communication, the parents are further asked whether the child’s functioning within this domain leads to problems in daily living.

Additionally, a single item question determines whether a child is bilingual or multilingual (‘Is your child bilingual or multilingual?’) with two response options: ‘Yes, my child speaks and/or understands multiple languages’ or ‘No, my child speaks and/or understands only one language’. Parents were asked to compare their child’s language to peers (‘How is your child’s language compared with peers of the same age?’) with response options: ‘Significantly better’, ‘Slightly better’, ‘Comparable to peers’, ‘Slightly worse’ and ‘Significantly worse’. The two questions were not included in the baseline data collection for wave 1.

Mental health problems

The Strength and Difficulties Questionnaire (SDQ)⁴⁵ is developed to screen for psychopathology in children aged 3–16 years and effectively detects emotional and behavioural problems among children. The questionnaire is used worldwide, and the parent-reported SDQ has generally demonstrated strong psychometric properties^{46 47} with similar findings reported for Danish children.⁴⁸ SDQ is completed by parents on how the child has been in the past 6 months and consists of 25 items divided into five subscales of five items each: ‘Emotional symptoms’, ‘Conduct problems’, ‘Hyperactivity/inattention’, ‘Peer relationship problems’ and ‘Prosocial behaviour’. The 25 questions ask about different positive and negative aspects of the child’s behaviour and are scored on a 3-point Likert scale: ‘Somewhat true’ is always scored as 1, but the scoring for ‘Not true’ and ‘Certainly true’ is either 0 or 2 depending on the items’ wording (ie, positively or negatively framed). A total difficulty score is calculated as the sum of scores of the conduct, hyperactivity, emotional and peer problems scales.⁴⁵

In addition, child temperament was assessed with a question inspired by a previous Danish study evaluating young children’s development and well-being.⁴³ Parents were asked ‘How will you assess your child’s temperament in general?’ and selected one of three response categories: ‘Has a strong temperament (if your child reacts to situations with frustration, anger, anxiety or withdrawal more frequently and/or intensely than other peers)’, ‘Has an average temperament (if your child’s temperament is similar to most peers)’ or ‘Has a mild temperament (if your child reacts to situations with frustration, anger, anxiety or withdrawal less frequently and/or intensely than other peers)’.

Social competences

The Social Competence Scale—Parent Version⁴⁹ is a 12-item measure of children’s positive social behaviours, including emotion regulation, prosocial behaviours and communication skills. As a brief measure of social competence among preschool children, the scale has shown adequate psychometric properties.⁵⁰ The questionnaire assesses two subscales; ‘Prosocial/Communication skills’ and ‘Emotional regulation skills’. Each item states a behaviour that the child might exhibit in a social setting and the parents assess how well each statement describes their child on a 5-point Likert scale: ‘Not at all’ (0), ‘A little’ (1), ‘Moderately well’ (2), ‘Well’ (3) and ‘Very Well’ (4). Subscale scores and the total score are calculated as the mean of responses.

Register follow-up

As all Danish residents are given a unique CPR number at birth or on immigration that is used for government services (eg, healthcare, education), we will be able to link questionnaire data of our cohort with data from nationwide registers for research purposes. We plan to link information of the 11 690 children from the baseline data collection with social and health registries in a pseudonymous way. The data linkage will include the Civil Registration System, the Danish Medical Birth Registry⁵¹ (captures birth-related information), the Danish National Patient Register⁵² (captures hospital admissions with diagnosis codes), the Danish National Prescription Registry⁵³ (captures drug prescriptions) and the Danish Psychiatric Central Register⁵⁴ (captures admissions due to psychiatric diagnoses). In addition, we will link with various social registers, including data on language assessment and national tests conducted in kindergarten and schools and school grades from the Danish Ministry of Children and Education, as well as social characteristics and well-being assessments collected by the health nurse. This linkage will allow us to investigate long-term psychological and social consequences of screen use among the cohort.

Data analysis and statistics

Data management

Data will be pseudonymised by a data management team shortly after the collection point. All participant-related data will be collected and securely stored on a server at the University of Southern Denmark, accessible by the project team only. Currently, the data will be stored until 2032, but we will apply for an extension to ensure longer-term storage. The collected information in this study is strictly confidential and both individual information and results will be pseudonymised. The data will be stored in line with policies and procedures consistent with the guidelines of University of Southern Denmark for technical solutions to collect, store and transfer data.

Questionnaire data will later be linked with register information from Statistics Denmark servers to enhance the study’s comprehensiveness. In cooperation with Statistics Denmark, a linked research database will be established where the data will be stored. This process is protected and regulated under Danish law. Statistics Denmark has high security and confidentiality requirements, including (1) CPR numbers are pseudonymised, (2) several steps of log-in procedures for researchers, (3) researchers are educated in data security and privacy requirements, (4) all activities are logged and (5) only aggregated data can be published.

The data will be archived in accordance with the Archiving Act (by the Danish National Archives Rigsarkivet) to ensure long-term preservation and future access for validation and further research. No data sharing is planned at this time, but data may be shared for statistical and scientific purposes in other studies, for example, with other research institutions and public authorities, on agreement.

Sample size and statistical power

In this study, we aimed to capture data for as many 4-year-old children in Denmark born during the specified periods as possible, to ensure a large and representative study population. Based on participation rates in similar large-scale studies,^{33 43 55} we expected a participation rate of 30%–50% for each wave. The sample size in the baseline sample (n=11 690) and in the first follow-up sample (n=5833) is well beyond any minimum threshold, which will result in a high degree of precision for the purposes of correlation and regression analyses. For instance, n=5000 provides more than 99% power to detect a small correlation of r=0.10. The precise analyses and expected effect sizes vary depending on the specific research question under examination, so a single power analysis would not represent all scenarios. However, given the large sample size, the study has a high statistical power to detect effects in the population, even in stratified samples.

Statistical analysis

To assess associations between amount, content, timing and context of screen use and psychological outcomes, we will apply generalised linear mixed models accounting for repeated measures. Baseline characteristics of the parents, child and home environment will be included in the analyses to control for potential confounding, and interaction effects of the child’s gender in addition to demographic characteristics of the family will be tested.

To examine potential mechanisms in the relationships between screen use and psychological outcomes, we will examine moderating effects of family and home characteristics in stratified analysis. In addition, we plan to perform latent class analysis and/or latent transition analysis to identify distinct patterns of screen use, potentially across time, based on duration, content, timing and contextual factors such as co-viewing and parental screen use. The latent structures will then be used as exposure variables in subsequent analyses to explore their differential impact on psychological outcomes. To disentangle direct and indirect effects, structural equation modelling will be used to explore whether sleep duration mediates the relationship between screen use and psychological outcomes.

Missing data will be quantified using descriptive statistics, from which it will be determined if missing data could bias results or reduce statistical power. To address missing data and preserve statistical validity, we will employ multiple imputations, which account for uncertainty by generating several plausible datasets and combining results.

Reverse causation remains a potential limitation, as children’s pre-existing psychological or development problems may influence their screen use. To address this, we will use longitudinal modelling techniques, including cross-lagged panel models, to examine directionality of effects. In addition, sensitivity analyses will assess the robustness of findings, including adjustments for missing data. This multimethod approach ensures a rigorous evaluation of pathways linking screen use to children’s mental well-being.

Different screen use characteristics may be analysed in the same models if including them simultaneously does not introduce multicollinearity. However, on identifying effect modification through variable interaction, we will stratify the analyses by the modifying variable to accurately estimate subgroup-specific effects and interpret heterogeneity. Data processing and analysis will mainly be conducted using R V.4.4.1.

Ethics and dissemination

The invitation letter elaborated on the study purpose and methods, the right to withdraw at any time, and contact information in case the invited participants had questions concerning the survey. The letter emphasised that participation was voluntary and by completing the questionnaire, participants consent to the processing of their personal data. Participants were also informed that individual data would be kept confidential and that their data would only be used for research. Recognising the sensitivity and potential stigma around screen time, this project ensured that all questions and communication were framed in a non-judgmental and non-stigmatising manner.

The study complies with national regulations regarding consent, data protection and ethics approval. The study was exempted from further ethical approval from the Regional Health Research Ethics Committees in the Capital Region in Denmark (no. F-24067129), as the questionnaire-based study is not classified as a health science research project under the Danish Committee Act. The local Data Protection Agency at the University of Southern Denmark also approved the data collection (no. 12.102), the linking of data to registers within the purpose of the study based on the EU general data protection regulation and ensured that all local confidentiality and privacy requirements were met.

All results from The Digital Child Study will be published in relevant peer-reviewed journals and presented at conferences, nationally and internationally. We will also share findings to the broader community with plain-language and illustrative summaries of main findings to ensure that findings reach stakeholders capable of implementing them.

Discussion

In this paper, we present the design, data collection methods and measures of a large-scale national longitudinal study: The Digital Child Study. The study aims to provide a comprehensive picture of screen use and psychological outcomes with repeated measurements among Danish preschool children aged 4–5 years and with planned long-term registry follow-up. Parents or primary caregivers of all Danish children born between specific months in 2020 were recruited, resulting in a population of 11 690 children in the first baseline data collection.

A major strength of the study is the establishment of the largest longitudinal cohort of preschool children in Denmark. The large sample size in the baseline sample of more than 11 000 children, in addition to an expected follow-up of 50% ensures the robustness of associations, even in stratified analyses. The longitudinal design offers a unique opportunity to investigate the temporal relationship between screen use and psychological development among preschool children, addressing one of the main limitations in the current literature. In addition, the comprehensive design enables the study to move beyond the typical focus on screen time, another main limitation in the current literature. By including questions related to content and timing, as well as questions on the broader context of the child’s screen use, encompassing family screen culture such as parental mediation strategies, attitudes, motivations and practices, we will be able to provide a complete overview of the complex factors involved in childhood screen use. Furthermore, the study employs several well-validated and reliable measures of cognitive and emotional development outcomes that are commonly used in early childhood research (eg, SDQ, BRIEF-P).

Another key strength of the study is the ability to prospectively follow a large sample of Danish children (n=11 690) in nationwide registers in years to come for mental and physical health diagnoses and social outcomes, allowing for almost complete long-term follow-up for key health outcomes. Danish health data registries are unique and of very high quality.⁵⁶ Data from the cohort will thereby be able to generate much-needed insights into the mechanisms underlying well-being issues and disorders in childhood, providing a platform for preschool-aged child health research that can serve as a founding for the development of targeted interventions for children and their families.

Despite its many strengths, the study also has some limitations. As in most epidemiological studies, misclassification due to self-reported data is a potential issue. The parent-reported data can be vulnerable to random or systematic misreporting. Social desirability bias could lead to underestimation of children’s screen time, and recall bias may impact the accuracy of responses regarding specific behaviours or experiences. In addition, selection bias due to selective entry into the study or loss to follow-up is also an inherent challenge, as participation is voluntary and, especially, the baseline and second follow-up questionnaires can be a demanding task for parents who may already be time poor (25 min). While we cannot fully document the bias arising from potential self-selection, we are mostly concerned if parents from socially disadvantaged backgrounds were less likely to participate. Finally, confounding is an inevitable source of bias in observational studies, which we cannot rule out. To minimise this bias, we will adjust for several relevant factors in our statistical analyses (gender, socioeconomic background, etc). However, residual confounding due to unmeasured or poorly measured confounders remains a potential concern.

In conclusion, The Digital Child Study will contribute to our understanding of the complex relationship between screen use and psychological development in preschool children. The longitudinal design offers a unique opportunity to identify patterns of screen use over time and their potential impacts, addressing key gaps in existing literature. By examining risk and protective factors, such as screen content, parental screen mediation and family environment, the study moves beyond a simplistic focus on screen time to offer a nuanced perspective on digital media’s role in early childhood. These findings can inform evidence-based guidelines for parents, caregivers and policymakers, supporting interventions aimed at promoting positive child development in an increasingly digital world.