Season of birth bias and anorexia nervosa: Results from an international collaboration

Abstract

The main implication of these findings is that season of birth may not play a significant part in the aetiology of AN.

Comparing effects: a reanalysis of two studies on season of birth bias in anorexia nervosa

“…The studies by Disanto et al [1] and Winje et al [2] were chosen as i) their conclusions differ, ii) they both have large samples and included information on power calculations, iii) Disanto et al [1] performed a meta-analysis (analysing the pooled sample), and iv) Winje et al [2] included samples from several continents on both the Northern and the Southern hemisphere, as well as a pooled analysis. A secondary aim or this paper is to discuss the results according to common conventions for interpreting effect sizes (Cohen’s categories [8]) and their practical implications.…”

“…To enable comparison of the studies by Disanto and colleagues [1] and Winje et al [2], the statistical analyses used in each study were applied to the samples from the other study. Disanto et al [1] performed a Walter and Elwood’s test [9] and chi-square analyses contrasting i) the first vs. the second half of the year (1df), ii) March-June vs. the rest of the year (1df), iii) September-October vs. the rest of the year (1df), and iv) March-June vs. September- October (1df).…”

“…Winje et al [2] performed a two-tailed chi-square test for contingency tables with known population parameter [10] to test for monthly deviations (11 df). The effect sizes reported were Cramêr’s V. This is a measure of the inter-correlation between variables, when there are more than two categories.…”

“…Their sample consisted of patients with AN collected from four previously published studies [3–6], and was compared to the national distribution of births retrieved from the UK Office for National Statistics. On the other hand, Winje and colleagues [2] concluded that their findings did not support a season of birth bias hypothesis. Their sample consisted of females with AN who were recruited from 16 centres in nine different countries, resulting in five samples which were compared to the distribution of births in the general population in the same areas, retrieved from the corresponding statistical bureaus.…”

“…A season of birth bias means that more patients than expected from the normal population are born during certain months; indicating this could be a marker of yet unknown causal factors for that disorder [1, 2]. Several studies have investigated season of birth bias in eating disorders [1–4].…”