title: Comparing effects: a reanalysis of two studies on season of birth bias in anorexia nervosa.
abstract: BACKGROUND
  Outcomes from studies on season of birth bias in eating disorders have been inconsistent. This inconsistency has been explained by differences in methodologies resulting in different types of effect sizes. The aim of the current study was to facilitate comparison by using the same methodology on samples from two studies with differing conclusions.
  
  
  METHODS
  The statistical analyses used in each study were applied to the samples from the other study and the resulting effect sizes, Cramêr's V and odds ratio (OR), were compared and discussed.
  
  
  RESULTS
  For both studies, the Cramêr's Vs ranged between 0.03 and 0.08 and the OR ranged between 0.85 and 1.31. According to common conventions, Cramêr's Vs below 0.10 and ORs below 1.44 are considered small.
  
  
  CONCLUSIONS
  As a marker of one or more potential risk factors, the observed effects are considered to be small. When reanalysed allowing for direct comparisons, studies with contrasting conclusions converge towards an absence of support for a season of birth bias for patients with AN.
authors.affiliation: Division of Mental Health and Addiction, Regional Department for Eating Disorders, Oslo universitetssykehus HF, Postboks 4956, Nydalen, 0424 Oslo, Norway.
authors.forename: Eirin
authors.initials: E
authors.lastname: Winje
authors.affiliation: Department of Psychology, University of Oslo, P.O. Box 1039, 0317 Oslo, Norway.
authors.forename: Anne-Kari
authors.initials: AK
authors.lastname: Torgalsbøen
authors.affiliation: Oslo Centre for Biostatistics and Epidemiology, Research Support Services, Oslo University Hospital, Ullevål, Oslo, Norway.
authors.forename: Cathrine
authors.initials: C
authors.lastname: Brunborg
authors.affiliation: Division of Mental Health and Addiction, Regional Department for Eating Disorders, Oslo universitetssykehus HF, Postboks 4956, Nydalen, 0424 Oslo, Norway.
authors.forename: Kristin
authors.identifier: 0000-0003-3078-3758
authors.initials: K
authors.lastname: Stedal
keywords: Anorexia nervosa; Eating disorders; Effect sizes; Methodology; Season of birth