Independent age-group differences versus longitudinal anthropometric changes in male adolescentsaged 16–18 years: an exploratory pilot study
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Abstract
Introduction: Independent age-group contrasts can describe age-related
anthropometric patterns, but they do not measure within-individual change and may
also reflect cohort composition and calendar time. Objective: To describe how far
a non-contemporaneous independent age-group contrast diverged numerically from
longitudinal change observed within the same cohort over a similar age interval in one
institutional sample. Methods: In this exploratory pilot study, 27 male boarding-school
students were followed from May 2023 to April 2025, and an independent group of
22 students recorded as approximately 18 years of age was assessed in April 2024.
Body height, body weight, seven skinfolds, body mass index, and secondary estimated
body-composition indicators were evaluated. Results: Mean longitudinal increases
were 3.69 cm for body height and 6.36 kg for body weight, whereas the corresponding
independent age-group differences were 4.44 cm and 14.18 kg. The discrepancy
was 0.75 cm for body height (95% confidence interval −3.38 to 4.88) and 7.81 kg
for body weight (−3.06 to 18.69). Chest skinfold showed the largest standardised difference between the two approximately 18-year-old groups. Conclusions: Because
the independent age-group difference and longitudinal change shared the same
baseline mean, their discrepancy is algebraically equivalent to the direct difference
between those two older groups and cannot isolate effects of study design from cohort,
maturation, or calendar-time effects. These results therefore provide an exploratory
demonstration that independent age-group contrasts may differ from observed withinindividual change; they do not evaluate comparative validity or equivalence of the two designs. The small class-based sample limits precision and generalisability.
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