title: Interpreting the beta geometric in comparative fecundability studies.
abstract: The beta distribution has often been used to describe variation in fecundability between couples when modelling the number of menstrual cycles required for a couple to achieve pregnancy. Groups are often compared for a condition or factor that could affect conception rates. However, when heterogeneity is present in each group, comparing group-specific parameters is not very informative on what any differences might imply for the individual couples. We present a quantile ratio that provides an individual level interpretation of any between-group differences in the beta distribution. Our results are illustrated on the two examples of Ridout and Morgan (1991, Biometrics 47, 1423-1433). The quantile ratio suggests a simplification in one of the examples.
authors.affiliation: Centre for Applied Statistics, Fylde College, University of Lancaster, U.K. R.Crouchley@lancaster.ac.uk
authors.forename: R
authors.initials: R
authors.lastname: Crouchley
authors.forename: A
authors.initials: A
authors.lastname: Dassios
mesh_terms: D001699:Biometry / Q000379:methods*; D003276:Contraceptives, Oral / Q000009:adverse effects; D005260:Female; D005298:Fertility; D006801:Humans; D008297:Male; D011247:Pregnancy; D012907:Smoking / Q000009:adverse effects; D013997:Time Factors
chemical_list: D003276:Contraceptives, Oral