title: Invited commentary: composite outcomes as an attempt to escape from selection bias and related paradoxes.
abstract: This commentary reviews the recent history of explanations to crossover paradoxes such as the birth weight and gestational age paradoxes, with a special emphasis on the current proposal by Kramer et al. in this issue of the Journal (Am J Epidemiol. 2014;179(3):361-367). We contend that the causal structure of these paradoxes is essentially identical to that of several well-known selection biases. We then consider the pros and cons of using composite outcomes to circumvent these selection biases.
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Hernán
authors.forename: Enrique F
authors.initials: EF
authors.lastname: Schisterman
authors.forename: Sonia
authors.initials: S
authors.lastname: Hernández-Díaz
mesh_terms: D003627:Data Interpretation, Statistical; D015340:Epidemiologic Research Design; D005260:Female; D005865:Gestational Age; D006801:Humans; D054238:Perinatal Mortality; D011247:Pregnancy; D047928:Premature Birth / Q000209:etiology*; D050497:Stillbirth
keywords: causal diagrams; composite outcomes; paradox; selection bias