Longitudinal research in Aotearoa New Zealand

How do people live? How do we change as we grow? What happens as we get older? If an event or experience occurs during childhood, will it have an affect in later life, and how? These are some of the questions that longitudinal studies are intended to help address.

Some may question why scientists undertake longitudinal research at all, when there are cheaper, faster designs for research studies. Some may argue that retrospective studies can collect data on life history in a more economical way. They may favour retrospective designs because they can get quick answers from participant recall; however, recall can be unreliable (especially over longer periods) and further, many types of observations (such as measurement of clinical measures or biomarkers) are not feasible in retrospective designs. Prospective collection of data in longitudinal studies, by contrast, can help minimise the risk of recall bias among research participants. A person's beliefs and understanding of the world evolves with age, and so a person may look back differently on an event from the way they experienced it at the time. Future-focused studies allow investigation of how the effects of exposures unfold over time and are influenced by other life events. This means that observational, multidisciplinary research is needed in order to begin to understand people and society.

Longitudinal studies can be used to characterise exposures, whether acute or chronic, and relate events to them in terms of sequence and timing. In addition, longitudinal findings can be used to describe trajectories across a lifespan, establishing the natural history of development, aging, or progression of a condition. However, at its core, longitudinal research helps answer holistic, deep questions about the complex interplay between various biopsychosocial factors that act at different points through life. Longitudinal research can help address questions about macrosocial issues; it is not just about epidemiology, it is about the lifecourse and understanding society. Human lives are brilliantly complicated, and no one researcher could claim to be an expert in all aspects of research on human development, health, and aging. This means that high impact, informative longitudinal research is a highly specialised area and requires skilled, knowledgeable, and versatile researchers. Some longitudinal studies can outlive the careers of researchers and new generations of researchers must be brought aboard to replenish their ranks. Researchers from differing areas of academic specialities must collaborate over long periods of time, observing at different stages of life and in different ways. Thus, substantial and sustained investment is required to prepare for and maintain successful longitudinal research. The costs are relatively high but the return is great in improving our knowledge about ourselves.

It is important to note that longitudinal studies are not without limitations. For example, the burden of participating in such research can be high, and a major threat to the success of any longitudinal study is participant attrition, or loss to follow-up, over time. This problem becomes more serious the longer a longitudinal study progresses. Another challenge is the fact that science is not static; in older longitudinal studies, research methods were based on the technology, standards, and knowledge of the day, meaning that investigators are required to be familiar with a broad array of research methods, both old and new. Older studies have their roots in scientific history; they were born in another time and used outdated methods. For example, early data collected in some of Aotearoa New Zealand's older longitudinal studies were saved and processed on punch cards. Data collected using older methods can often be adapted to meet new understandings while maintaining longitudinal continuity of concepts and understanding; there is a fine balancing act between older and more modern methods, and all add value and augment each other. Integrating these is key to the success of longitudinal studies.

Aotearoa New Zealand has a thriving longitudinal research community, as evidenced by the number of high-quality and seminal papers published in this Special Issue. However, society changes rapidly and we need to keep up with these changes in order to understand the cohort and period effects that impact upon how each generation lives. Research teams have been building expertise and human capacity within existing studies in recent decades, but ongoing investment in longitudinal research is needed to maintain these cohorts and continue follow-up. Crucially, in order to continue to thrive as a research community, we must replenish and maintain an active, well-resourced and well-supported research workforce, and replenish and maintain the cohorts that are so integral to our longitudinal studies. To achieve this, a national infrastructure for support of longitudinal studies is required in order to maintain these cohort studies and select and develop new ones. Moreover, within Aotearoa New Zealand, we need to cooperate between longitudinal studies, since studies should serve to verify findings and make up for each other's limitations. This means that incentives for collaboration between studies are also needed, along with incentives to make collaboration by international experts more attractive and feasible.

High interest and a large number of quality submissions has meant that we have divided this Special Issue into two parts, and this first part includes six papers. Three of these are reports from investigators with the Growing up in New Zealand Study, our largest longitudinal study of child development.

Firstly, Morton and colleagues review findings from the considerable body of work contributed by that study on the issue of inequality. They discuss the disproportionate impacts on wellbeing for Māori and Pacific children, and how socioeconomic hardship can lead to intergenerational continuity in inequality (Morton et al. 2022). A second report from Growing Up in New Zealand, by Atatoa Carr et al., tackles the challenge of understanding the self-identification of ethnicity in Aotearoa New Zealand. Their findings have important implications for understanding how people identify themselves, and how this can change or remain stable over time. This has implications both for how we understand society and for how we should conduct longitudinal research in Aotearoa New Zealand (Atatoa Carr et al., 2022). Next, Paine and colleagues discuss the importance of protecting participants in longitudinal research. They discuss the potential that following kaupapa Māori approaches hold for longitudinal research in Aotearoa New Zealand, and give the example of how Kaitiaki guardianship principles have been applied in the Growing Up in New Zealand cohort in order to protect participants and remain true to Te Tiriti o Waitangi (Paine et al. 2022). Moving to some older cohort studies, Ruiz et al. discuss some of the issues in using early findings from the Dunedin Multidisciplinary Health and Development Study and the Christchurch Longitudinal Study to help understand the issues of today, using the example of oral health. Oral health is an under-investigated issue in a majority of longitudinal studies. Oral health among Aotearoa New Zealand children remains problematic, and this study shows how oral health at age 5 in the Dunedin and Christchurch cohorts roughly mirror the oral health status of 5 year old children today (Ruiz et al., 2022). Signal and colleagues report on sleep at the very beginning of life, using findings from the Moe Kura study, a study grounded in Kaupapa Māori methodology. They report rich intergenerational data on sleep among mothers during pregnancy, around the time of birth, and in the years following, as well as sleep among their children. They report important findings on the value of sleep for child health, and on the inequity in good sleep (Signal et al., 2022). Finally, Milne discusses the Integrated Data Infrastructure, or IDI, a system of compiling deidentified administrative data for the whole population of Aotearoa New Zealand, and gives examples of how this has been applied to enable longitudinal research. The paper discusses the challenges with the IDI, including its limitations and important ethical governance issues with the use of the data (Milne, 2022). These are approximately half the planned articles on longitudinal research in Aotearoa New Zealand. In the second part of this special issue we will bring further editorial commentary from prominent researchers in longitudinal research on the findings and future of longitudinal research in Aotearoa New Zealand.