Original Research Article When and for Whom are Relative Age Effects Important? Evidence from a Simple Test of Cardiorespiratory Fitness SCOTT VELDHUIZEN, 1,2,3 * TERRANCE J. WADE, 3 JOHN CAIRNEY, 2,3 JOHN A. HAY, 3 AND BRENT E. FAUGHT 3 1 Social and Epidemiological Research Department, Centre for Addictions and Mental Health, Toronto, Ontario, Canada 2 Department of Family Medicine, McMaster University, Hamilton, Ontario, Canada 3 Department of Health Sciences, Brock University, St. Catharines, Ontario, Canada ABSTRACT: Objectives: When individuals of different ages are combined into a single group and an ability that varies with age is measured, younger individuals are disadvantaged. This phenomenon is known as a relative age effect (RAE) and has been shown to be widespread in sport and education. Methods: In this article, we examine RAEs in a large group of children tested repeatedly on the 20-m shuttle run, a common test of cardiorespiratory fitness. Following up on an earlier study that measured change with age, we add a growth curve model for change in variance, which makes it possible to derive RAEs for individuals of different ages and ability levels. Results: Results show that a 1-year difference in age is associated with a change in performance of about 0.2 stand- ard deviations. For 1-year age groups, this gives rise to ranking errors of 4 percentile ranks or less. We also show, how- ever, that these relatively small ranking errors are capable of producing large age differences within groups identified as exceptional. Depending on the level of ability required for selection, children born in the first quarter of the year can be expected to outnumber those born in the last by 1.5 times, 2 times, or more. This finding is consistent with previously reported variation in RAEs at different performance levels. Conclusions: Results imply that RAEs are likely to be of relatively minor concern when people are graded or ranked but can produce substantial inequities and misclassifications when people with extremely high or extremely low ability levels undergo selection. Am. J. Hum. Biol. 00:000–000, 2014. V C 2014 Wiley Periodicals, Inc. INTRODUCTION Measures of child development or performance are typi- cally interpreted through a comparison to peers. This may take the form of direct comparisons among children of the same sex and similar age, or a comparison of performan- ces to standards developed from a normative sample. In formal screens for developmental delay and in assess- ments for high or low functioning in education or ath- letics, it is common practice to group children by age and apply decision rules derived ultimately from a normative sample. In athletics, children are similarly grouped by age and then assessed against each other or against a common standard. This pooling of individuals across an age range creates one or two important problems. In school settings and organized sport, it is common to group children by chrono- logical annual age groups. A year is a long time in child- hood and adolescence, and, within a grade, there will be systematic differences in development and accrued expe- rience between individuals of different ages. In general, the oldest individuals in each group enjoy considerable advantages due to their greater experience and maturity, and this is reflected in better performance in athletics and academics (see Cobley et al., 2009a; Musch and Grondin, 2001; Wattie et al., 2008;). Phenomena of this type are known as relative age effects (RAEs), and they should be expected whenever age and ability are related and people of different ages are grouped together. Many mechanisms underlying RAEs have been pro- posed (reviewed in Musch and Grondin, 2001). Roles have been proposed for teacher or coach interactions, expecta- tions, access to training, methodological biases (Delorme et al., 2010), season of birth, and the compounding of advantage from year to year (mechanisms are reviewed in Cobley et al., 2009a,b; Musch and Grondin, 2001; Wattie et al., 2008). RAEs have also been shown to persist into adulthood, after any maturational advantage is gone: In adult elite sports teams and in university admissions, for example, individuals born early in the year are often over- represented (e.g., Addona and Yates, 2010; Bedard and Dhuey, 2006; Schorer et al., 2009). This is most success- fully explained by the maturation-selection hypothesis (Cobley et al., 2009a,b), which argues that an initial selec- tion earlier in life provides increased opportunities and training. Other important theoretical and practical work has explored the complex effects arising from annual vari- ation and different types of age group structures (Schorer et al., 2013). At the time of this initial selection, however, and in gen- eral settings such as school classes, a very simple model of RAEs may be adequate. RAEs can be understood as the mathematically inevitable result of imposing age groups in a context where ability varies continuously with age. In a previous article, we considered RAEs in a general set- ting for one measure of cardiorespiratory fitness, the 20-m shuttle run (Veldhuizen et al., in press a), and argued that RAEs arose simply from the underlying relationship between age and ability, with no net effect for other Contract grant sponsor: The Canadian Institute of Health Research; Contract grant number: 66959 *Correspondence to: Scott Veldhuizen, Social and Epidemiological Research Department, Centre for Addiction and Mental Health, 33 Russell St., Suite T308, Toronto, Ontario, M5S 2S1, Canada. E-mail: scott.veldhuizen@camh.ca Received 17 December 2013; Revision received 4 March 2014; Accepted 15 March 2014 DOI: 10.1002/ajhb.22550 Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com). V C 2014 Wiley Periodicals, Inc. AMERICAN JOURNAL OF HUMAN BIOLOGY 00:00–00 (2014)