Validation of the Japanese Version of the Rating of Fatigue Scale

Abstract

Background

Fatigue research remains important across a wide range of fields. In Japan, fatigue has become a major social issue, and research using Japanese population data is expected to contribute to both practical interventions and theoretical advances. Although several Japanese scales for assessing fatigue have been developed, brief instruments designed to measure fatigue during ongoing activity are limited. Therefore, this study translated the Rating of Fatigue (ROF) scale into Japanese and examined the validity of the Japanese version (ROF-J).

Methods

First, the original ROF was translated into Japanese and back-translated. The comparability of language and similarity of interpretability between the original ROF and the ROF-J were evaluated. These procedures were conducted between July and October 2024. Second, the face validity and concurrent validity of the ROF-J were examined in a questionnaire survey of 85 university students at Tohoku University, Japan, in October 2024. Third, convergent validity was examined by comparing ROF-J scores before and after physical exercise among 35 university sports team members at Tohoku University, Japan, in November 2024.

Results

As a result of the comparison between the original ROF and the ROF-j, the comparability of language and similarity of interpretability were generally within acceptable limits. According to the examination of face validity of the ROF-J, participants generally agreed that the ROF-J measured fatigue, supporting its face validity. In addition, ROF-J scores showed a significant positive correlation with scores on an established Japanese fatigue questionnaire, supporting concurrent validity. The survey for examining convergent validity indicated that ROF-J scores increased significantly after exercise.

Conclusions

These findings indicate that the ROF-J demonstrates adequate validity. The scale provides a brief and practical Japanese instrument for assessing subjective fatigue during ongoing activity and may facilitate both applied fatigue assessment and future fatigue research in Japan.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s40798-026-01108-8.

Key Points

• We translated the Rating of Fatigue scale into Japanese and assessed the face validity, concurrent validity, and convergent validity of the Japanese version of the Rating of Fatigue scale (ROF-J).

• The survey results of university students indicated that they understood the purpose of the ROF-J and that the scale was associated with another fatigue measurement tool, supporting the face validity and concurrent validity of the scale. Also, an increase in the ROF-J score after physical exercise demonstrated the convergent validity of the scale.

• The ROF-J is a brief and practical Japanese instrument for assessing subjective aspects of fatigue, contributing to the advancement of fatigue research in Japan and supporting the use of quantitative methods in applied settings.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s40798-026-01108-8.

Background

Fatigue has been studied in medicine, psychology, sports science, ergonomics, and occupational health, and research on fatigue remains important in each field. In medical sciences, researchers are increasingly clarifying the biological mechanisms underlying fatigue [1], which may improve understanding of human homeostatic regulation. In psychology, recent studies show that fatigue affects decision-making in various contexts, such as risk-taking behavior [2] and moral judgment [3]. Considering fatigue as a factor that modulates decision-making may help develop more accurate models of human cognition and behavior. In sports science, many studies have examined the relationship between subjective fatigue and physical performance [4], which may clarify mechanisms behind performance fluctuations. Overall, progress in fatigue research is expected to support the development of related academic fields. Given these broad implications, it is also essential to consider how fatigue manifests and is addressed within specific populations.

Research on fatigue in the Japanese population is important. Aritake et al. [5] conducted an epidemiological survey and found that 17.2% of Japanese individuals consider themselves easily fatigued, and 13.2% experience residual fatigue, which is a feeling of fatigue that persists with no signs of recovery. These results indicate that a significant proportion of people in Japan experience persistent fatigue. Many individuals also lack adequate sleep, which is necessary for recovery from fatigue. Adult sleep recommendations generally emphasize the importance of securing sufficient sleep, with international recommendations suggesting at least seven hours per night [6, 7] and Japanese guidelines recommending at least six hours [8]. Nevertheless, a national survey by the Ministry of Health, Labour and Welfare reported that 38.5% of men and 43.6% of women sleep less than six hours per night on average, and 25.1% of Japanese adults do not feel adequately rested from sleep [9]. Given that insufficient sleep increases fatigue [10–13], these findings suggest that fatigue related to insufficient sleep may be common among Japanese adults.

In Japan, accumulated fatigue from overwork has resulted in fatal outcomes such as karoshi (death from overwork), which is a serious social issue [14]. Since 1988, when lawyers representing bereaved families established the National Defense Counsel for Victims of Karoshi [15], both public and private sectors have implemented various countermeasures. In 2014, the Japanese government enacted the Act on Promotion of Preventive Measures against Karoshi. Based on this law, the Ministry of Health, Labour and Welfare has introduced several initiatives, including research on overwork-related mortality, educational programs in junior and senior high schools, a national awareness month, symposia to promote prevention, and recurrence-prevention guidelines for employers [16]. Despite these efforts, from 2022 to 2023, overwork-related deaths from cerebrovascular and cardiovascular diseases among private-sector employees increased from 54 to 58, while suicides and attempted suicides associated with overwork-related mental disorders rose from 67 to 79 [17]. These findings show that fatigue is highly prevalent in the Japanese population. Therefore, research on fatigue using Japanese population data is expected to help address fatigue-related social problems in Japan and advance fatigue research more broadly.

Recent nationally representative data on the prevalence of fatigue stratified by age and socioeconomic status are limited in Japan. However, indicators related to sleep and overwork suggest that the burden associated with fatigue may not be uniformly distributed across population groups. In the 2024 National Health and Nutrition Survey, the proportion of individuals reporting insufficient rest from sleep was highest among adults in their 40 s in both men and women (30.0% and 28.4%, respectively) [17]. The proportion sleeping less than six hours per night was highest among adults in their 50 s (48.3% of men and 49.1% of women) [17]. These findings suggest that exposure to short and nonrestorative sleep, both of which are associated with fatigue, may be greater among middle-aged adults; however, fatigue itself was not directly assessed in this survey. Occupational variation is also observed in administrative data on deaths recognized as related to overwork. According to the 2023 White Paper on Measures to Prevent Karoshi, road freight transport had the largest number of fatal cases among compensated cerebrovascular and cardiovascular disease cases, with 16 deaths [18]. Social insurance, social welfare, and care services had the largest number of fatal cases among compensated mental disorder cases, with three deaths [18]. These figures indicate that severe health outcomes related to overwork have been recognized more frequently in certain industries. However, they represent counts of compensated fatal cases rather than mortality rates based on the number of workers in each industry, and fatigue itself was not assessed. Therefore, this data cannot establish that the prevalence of fatigue or individual-level risk is highest among workers in these industries.

The functional purpose of fatigue is to suppress or terminate an individual’s activity [19, 20]. This occurs through two pathways. The first involves peripheral mechanisms, where biochemical changes within the muscle reduce activity independently of the central nervous system [21]. The second involves central mechanisms, where neurochemical changes produce subjective experiences such as tiredness and reduced motivation, leading individuals to voluntarily stop activity [19]. Both pathways are essential for achieving the functional purpose of fatigue [22]. In this study, we focused on the subjective experience of fatigue and defined fatigue as “an overwhelming sense of tiredness, lack of energy and a feeling of exhaustion, associated with impaired physical and/or cognitive functioning” [23].

Several Japanese scales for measuring fatigue have been developed. Kobayashi et al. [24] created the Subjective Fatigue Scale for Adolescent Students, a 24-item self-report measure with six factors for adolescents. The Working Group for Occupational Fatigue introduced the Jikaku-sho Shirabe [25], a 25-item scale assessing temporal changes in work-related fatigue. Sugaya et al. [26] translated the Multidimensional Fatigue Inventory (MFI) [27] into Japanese, resulting in a 20-item, five-factor version. Although these scales include many items, they may not be suitable when participants must respond during ongoing physical activity, work, or when they are highly fatigued.

To address this issue, two complementary approaches are available: (a) using physiological indicators as objective measures of fatigue, or (b) employing subjective rating scales with fewer items. For the first approach, several physiological markers (such as electroencephalography (EEG) [28] and heart rate variability (HRV) [29] have been proposed as reliable indices of fatigue. However, measuring and analyzing these indicators often require specialized expertise and advanced equipment, which limits their use to specific research settings and investigators. In contrast, scales with fewer items assess subjective fatigue without specialized instruments or technical knowledge, making them more feasible in diverse research contexts. Considering practicality and accessibility, developing concise scales for assessing subjective fatigue is both necessary and meaningful.

In this study, we translated the Rating of Fatigue scale (ROF) [30] into Japanese to develop a fatigue measurement scale that is easy for Japanese respondents to use, and we examined its validity. The ROF is a single-item scale originally developed in English to assess subjective fatigue.

Although some scholars have argued that single-item scales are inferior to multi-item scales in terms of reliability [31] and validity [32], single-item measures can be equally effective, or even superior under the condition where the construct to be measured is simple. When a multi-item scale is used to assess a simple construct, respondents may perceive the questions as repetitive and redundant, which can evoke discomfort or disengagement [33]. Employing a single-item measure can reduce this burden. In this study, fatigue is defined based on subjective experience, without dichotomies such as physical versus mental or central versus peripheral, and without reference to objective performance decrements. The focus is on whether individuals report a subjective sense of fatigue. Therefore, fatigue is treated as a unidimensional and simple construct, making a single-item scale appropriate.

In this study, we translated the ROF into Japanese and examined its face validity using procedures from the development of the French version of the ROF [34]. We also assessed the concurrent validity of the Japanese ROF by analyzing its associations with the Jikaku-sho shirabe [25], a widely used fatigue scale in Japan. To test convergent validity, we compared Japanese ROF scores before and after physical exercise.

For face validity, we hypothesized that providing instructions would make the Japanese ROF more comprehensible to participants. In the original ROF, instructional text led to significantly higher scores for the item “the ROF measures fatigue” and significantly lower scores for “the ROF measures exertion” [30]. The French version showed similar results, with instructional text increasing scores for “the ROF measures fatigue” [34]. These findings indicate that instructional text improves the face validity of the ROF. Therefore, we predicted that including instructional text would help participants understand the scale's intended purpose. For concurrent validity, we expected significant positive correlations between ROF-J scores and the Jikaku-sho shirabe subscales. Because the Jikaku-sho shirabe measures symptoms that often occur with fatigue, such as sleepiness, we anticipated it would covary with the ROF, which measures fatigue. For convergent validity, we hypothesized that Japanese ROF scores would be significantly higher after physical exercise than before, based on Micklewright et al. [30], who found increased ROF scores following physical activity.

Methods

Study Design

Following the design of Brownstein et al. [34], this study included three phases: translation of the ROF scale into Japanese, assessment of face validity and concurrent validity, and assessment of convergent validity. For all phases, we obtained ethical approval from the Institutional Review Board of the author’s university and secured written informed consent from all participants.

Phase 1: Translation of the ROF Scale into Japanese

Phase 1 included three stages: translation of the original ROF into Japanese, back-translation of the Japanese version into English, and comparison of the back-translated versions with the original. This procedure was based on Brislin’s back-translation method [35], which uses comparison between the original and back-translated versions to identify and resolve semantic discrepancies in the translated version. Subsequent methodological guidelines for the cross-cultural adaptation of self-report measures have also recommended procedures involving forward translation, back-translation, and comparison with the original version [36–38]. Thus, the present translation procedure was consistent with established approaches for enhancing the semantic and conceptual equivalence of translated instruments. In the first stage, two graduate students, who are native Japanese speakers and major in psychology, independently translated the original ROF into Japanese. One of them was naïve to the purpose and hypothesis of this study, while the other was not. Three native Japanese speakers, including the two graduate students who did the initial translation and a psychologist who are not naïve to the purpose and hypothesis of this study, then resolved any differences and produced a single consolidated version. The first stage was conducted in July 2024. In the second stage, three native English speakers proficient in Japanese back-translated the Japanese version into English (see Additional file 1). One of the back-translators was a Japanese language school teacher, another was a graduate student, and the third was an undergraduate student. They were unaware of the study’s purpose, unfamiliar with the original ROF. They completed the back-translation by September 2024. In the final stage, four English speakers including three graduate students and an assistant professor compared the three back-translated versions with the original. They were unaware of the study’s purpose and unfamiliar with the original ROF. They independently rated the comparability of language and similarity of interpretability using a 7-point Likert scale (1 = Extremely comparable/similar, 7 = Not at all comparable/similar), following Sperber et al. [39]. Comparability of language refers to the formal similarity of words, phrases, and sentences between the back-translation and the original, while similarity of interpretability refers to the extent to which both versions would produce the same attitude response even if the wording differed. We conducted data collection and analysis for the final stage from September to October 2024.

Phase2: Assessment of Face Validity and Concurrent Validity

In Phase 2, we conducted a questionnaire survey to assess the face validity and concurrent validity of ROF-J. A priori power analysis using G*power 3.1 [40] indicated a required sample size of 82 participants (correlation: point biserial model, two-tailed, α = 0.05, 1 – β = 0.80, |ρ|= 0.30). A total of 85 Japanese university students from Tohoku University (36 men, 49 women; mean age 20.27 ± 1.63 years) participated in the survey via Google Forms. We recruited participants through lectures in Tohoku University and by requesting individual responses online. Participants were eligible if they were able to complete the Japanese-language questionnaires and provided informed consent. No specific exclusion criteria relating to health status or demographic characteristics were applied. We conducted participant recruitment and data collection in October 2024.

To assess face validity, we translated the six items developed by Micklewright et al. [30] into Japanese and included them in the survey: “The ROF represents fatigue”, “The ROF represents exertion”, “Descriptive components make the scale easy to understand”, “Descriptive components assist in deciding upon a rating”, “Overall scale is difficult to understand”, and “The instruction is helpful in understanding the scale.”

For concurrent validity, we used the Japanese version of the ROF scale and the Jikaku-sho shirabe [25], a Japanese questionnaire developed by the Industrial Fatigue Research Committee of the Japanese Society for Occupational Health to track changes over time in fatigue levels associated with work. The Jikaku-sho shirabe consists of five subscales: drowsiness (e.g., “I feel giving a yawn”), instability (e.g., “I feel nervous”), uneasiness (e.g., “I feel heavy in the head”), dullness (e.g., “I feel stiff in the neck and shoulders”), and eyestrain (e.g., “I feel dry eyes”). Each subscale contains five symptoms.

In the survey, participants first rated their agreement with the face validity items (excluding “The instruction is helpful in understanding the scale”) using a 5-point Likert scale (0 = Strongly disagree to 4 = Strongly agree) and completed the ROF-J without instructions. Next, they received the ROF instructions, rated their agreement with all face validity items (including “The instruction is helpful in understanding the scale”) on the same 5-point scale, and completed the ROF-J with instructions (see Additional file 1). Finally, participants rated the intensity of each symptom on the Jikaku-sho shirabe using a 5-point Likert scale (1 = Not at all to 5 = Extremely).

Phase3: Assessment of Convergent Validity

In Phase 3, to assess the convergent validity of ROF-J, we conducted a field survey with university sports teams during regular practice sessions in November 2024. Prior to the survey, we used G*power 3.1 [40] to calculate the required sample size, determining that 34 participants were needed (means: difference between two dependent means [matched pairs, two-tailed], α = 0.05, 1 – β = 0.80, d = 0.50). We recruited 20 male students from a men's volleyball team and 17 female students from a women's lacrosse team in Tohoku University. The final analysis included 35 students (18 men, 17 women; mean age 20.46 ± 1.87 years), as two students were excluded due to incomplete responses. No specific exclusion criteria relating to health status or demographic characteristics were applied.

We asked participants to rate their fatigue intensity using ROF-J on paper before and after their team practices. The men’s volleyball team practiced in a gym from 7:00 p.m. to 9:00 p.m., starting with basic drills (e.g., serving and receiving) and then moving to match-style play in small sub-teams. The women’s lacrosse team practiced at an athletic stadium from 6:00 a.m. to 9:30 a.m., beginning with basic drills (e.g., passing and shooting) and then continuing with match-style play in sub-teams. The youngest lacrosse members did not participate in matches and instead continued with basic drills. All team members, regardless of practice content, completed the questionnaire.

Statistical Analysis

We performed statistical analysis using jamovi 2.6.26 and set the significance level at 0.05 (two-tailed). In Phase 2, to assess gender differences in ROF-J scores, we conducted an independent-samples t-test and hypothesized no significant difference between men and women. For face validity, we conducted paired-sample t-tests on each face validity item (except “The instruction is helpful in understanding the scale”), comparing responses between the condition without instructions and the condition with instructions for ROF-J. We hypothesized that scores would be higher with instructions than without. We also conducted a one-sample t-test to determine whether the score on the item “The instruction is helpful in understanding the scale” was significantly higher than 2, the midpoint of the scale, and hypothesized that the score would be significantly greater than 2. To assess concurrent validity, we performed correlation analyses between ROF-J scores and each subscale score of the Jikaku-sho shirabe, hypothesizing a positive correlation. In Phase 3, for convergent validity, we conducted a repeated measures mixed ANOVA and a post hoc test with Bonferroni correction to examine the effect of measurement time (before or after practice) and team affiliation (men’s volleyball or women’s lacrosse) on ROF-J scores, hypothesizing that ROF-J scores would be higher after sports practice than before.

Results

Phase1: Translation of the ROF Scale into Japanese

To determine whether the ROF-J matched the original ROF, we calculated the mean comparability of language and similarity of interpretability scores given by the four English speakers for each paragraph or sentence. Four paragraphs had mean comparability of language scores greater than 3, and one sentence had both mean comparability of language and similarity of interpretability scores greater than 3. Following the threshold suggested by Sperber et al. [39], we revised the sentence for which both mean scores exceeded 3 to bring the meanings of the original ROF and ROF-J closer.

Phase2: Assessment of Face Validity and Concurrent Validity

Descriptive statistics for each scale are presented in Table 1. An independent-samples t-test on ROF-J score revealed no significant difference between men (mean score 3.89 ± 1.85) and women (mean score 4.57 ± 2.01) in ROF-J score.

Table 1.

Descriptive statistics for the Japanese ROF scale and the face-validity items

		Without the instruction		With the instruction
		Mean	SD	Mean	SD
The Japanese ROF scale				4.28	1.99
The face-validity items
The ROF represents fatigue		2.93	0.57	2.96	0.59
The ROF represents exertion		2.22	1.00	2.42	1.00
Descriptive components make the scale easy to understand		2.80	0.91	2.92	0.74
Descriptive components assist in deciding upon a rating		2.96	0.75	3.04	0.66
Overall scale is difficult to understand		1.53	0.92	1.61	0.94
The instruction is helpful in understanding the scale				2.94	0.98

ROF, Rating of Fatigue; SD, standard deviation

A paired-sample t-tests on each face validity item (Fig. 1) revealed that the differences in scores for the items “The ROF represents fatigue” (t (84) = − 0.58, p = 0.57, d = − 0.06), “Descriptive components make the scale easy to understand” (t (84) = − 1.56, p = 0.12, d = − 0.17), “Descriptive components assist in deciding upon a rating” (t (84) =− 0.93, p = 0.36, d = − 0.10), and “Overall scale is difficult to understand” (t (84) = − 1.00, p = 0.32, d = − 0.11) were not statistically significant between the two conditions. However, the score for “The ROF represents exertion” was significantly higher in the condition without instructions than in the condition with instructions (t (84) = 2.45, p < 0.05, d = 0.27). A one-sample t-test showed that the score was significantly higher than the midpoint (t (84) = 8.85, p < 0.001, d = 0.96).

Fig. 1.

The mean score on each face-validity item measured via Likert scale. Note. Light-gray bars represent the mean score on each face-validity item in the condition without the instruction. Dark-gray bars represent the mean score on each face-validity item in the condition with the instruction. Error bars depict standard deviation. The significance level α was 0.05. “n.s.” = the difference between the conditions was not significant

A correlation analyses between ROF-J scores and the subscale scores of the Jikaku-sho shirabe showed significant positive correlations between ROF-J scores and all subscale scores (see Table 2 for the full correlation matrix).

Table 2.

Pearson correlation coefficient between the ROF score and the Jikakusho-Shirabe score

Subscales	r	95%CI
Drowsiness	.64	.50–.75
Instability	.43	.24–.59
Uneasiness	.43	.24–.59
Dullness	.25	.04–.44
Eyestrain	.32	.11–.50

95%CI = 95% confidence interval

ROF, Rating of Fatigue; SD, standard deviation; r, Pearson correlation coefficient

Phase3: Assessment of Convergent Validity

A two-way mixed ANOVA showed a significant main effect of measurement time (F (1, 33) = 60.22, p < 0.05, η² = 0.43), no significant main effect of affiliation (F (1, 33) = 1.68, p = 0.20, η² = 0.02), and no significant interaction between measurement time and affiliation (F (1, 33) = 0.14, p = 0.72, η² = 0.00). A post hoc test with Bonferroni correction indicated that ROF-J scores were higher after practice than before (Fig. 2; t (34) = 7.76, p < 0.05).

Fig. 2.

The mean ROF score before and after practice. Note. Light-gray bars represent the men’s volleyball team’s mean ROF score. Dark-gray bars represent the women’s lacrosse team’s mean ROF score. The significance level α was 0.05. * p < 0.05. “n.s” = the difference between the teams was not significant. Error bars depict standard deviation

Discussion

In this study, we developed ROF-J and assessed its face, concurrent, and convergent validity using questionnaire surveys. The results generally supported the validity of ROF-J.

In phase 1, we translated the original version of ROF [30] into Japanese. English speakers then assessed the comparability of language and similarity in interpretability between the back-translated version and the original ROF, following procedures similar to those used for the French version [34]. Sperber et al. [39] state that if an item’s comparability of language and similarity of interpretability exceed 3, it is considered problematic. Based on this criterion, only the item “moderately fatigued” in the original ROF required revision. This issue likely resulted from a mismatch in the degree of fatigue expressed by the corresponding adjectives in the original ROF and ROF-J. To address this, we consulted an English speaker proficient in Japanese and revised the wording to ensure that the level of fatigue in ROF-J matched that of the original ROF. As in the original ROF, a visual illustration indicating the level of fatigue for “moderately fatigued” was included. Therefore, it is unlikely that interpretation of the ROF differs between respondents using the original and Japanese versions.

In phase 2, we assessed the face validity of the ROF-J using procedures similar to those for the original ROF [30] and the French version [34]. Consistent with previous versions, we found no significant differences between conditions for the items “descriptive components make the scale easy to understand”, “descriptive components assist in deciding upon a rating”, and “overall scale is difficult to understand.” These findings indicate that the descriptive components and overall comprehensibility of the scale were maintained in the ROF-J.

For the item “the instructions are helpful for understanding the scale”, which was included only in the instruction-present condition, the score was significantly higher than the theoretical midpoint, indicating that the instructions improved the comprehensibility of the scale.

We found no significant differences between conditions for the item “the ROF represents fatigue.” However, scores for “the ROF represents exertion” were significantly higher in the instruction-present condition. These findings differ from those of the original ROF and French ROF. Although the original ROF authors emphasized a conceptual distinction between fatigue and exertion, it may be difficult to separate these experiences subjectively because tasks involving high exertion often lead to fatigue. Therefore, using the scale with a strict distinction between fatigue and exertion may require further consideration.

In phase 2, we assessed the concurrent validity of ROF-J by examining its associations with the Jikaku-sho shirabe, a well-established Japanese fatigue scale. Significant positive correlations between ROF-J scores and each Jikaku-sho shirabe subscale score support the concurrent validity of ROF-J.

In phase 3, we assessed the convergent validity of the Japanese ROF by comparing scores before and after physical exercise. Scores on the Japanese ROF were significantly higher immediately after physical exercise. This result aligns with previous studies reporting increases in ROF scores with continued physical exercise [30, 34] and supports the convergent validity of the scale.

Study Limitations and Future Directions

This study has several limitations. First, the analysis of convergent validity focused only on associations with physical exercise. Future research should examine whether the ROF-J is sensitive to mental fatigue and to milder levels of fatigue than those typically experienced during sport by comparing scores before and after cognitive tasks, desk-based work, and other repetitive, low-intensity activities. Second, the sample consisted only of university students. Further studies should examine whether ROF-J is applicable to samples with a wider range of ages and characteristics such as workers, children, and clinical populations. Third, the examination of concurrent validity did not include associations between ROF-J and objective indicators of fatigue, such as performance decrements or physiological measures. Future research should evaluate concurrent validity by examining the relationship between ROF-J and objective fatigue-related indices.

Conclusions

In summary, ROF-J showed adequate validity. This study offers a brief and practical Japanese instrument for assessing subjective aspects of fatigue, contributing to the advancement of fatigue research in Japan and supporting the use of quantitative methods in applied settings.

Abbreviations

ROF

The Rating of Fatigue scale

ROF-J

The Japanese version of the Rating of Fatigue scale

Author Contributions

All authors collected and analyzed the data, and wrote, read, and approved the final version.

Funding

This study was partially funded by Tohoku University Advanced Graduate School Research Initiative for International Scholarly Excellence Program.

Data Availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics Approval and Consent to Participate

For the study, we obtained ethical approval from the ethics committee of Graduate School of Arts and Letters Tohoku University (approval number: 20240813) and secured written informed consent from all participants. The study was performed in accordance with the ethics outlined in the Declaration of Helsinki.

Consent for Publication

Not applicable.

Competing interests

Kaho Suzuki and Takashi Arai declare that they have no competing interests.