title: Collider Bias Is Only a Partial Explanation for the Obesity Paradox.
abstract: "Obesity paradox" refers to an association between obesity and reduced mortality (contrary to an expected increased mortality). A common explanation is collider stratification bias: unmeasured confounding induced by selection bias. Here, we test this supposition through a realistic generative model.
  
  
  
  We quantify the collider stratification bias in a selected population using counterfactual causal analysis. We illustrate the bias for a range of scenarios, describing associations between exposure (obesity), outcome (mortality), mediator (in this example, diabetes) and an unmeasured confounder.
  
  
  
  Collider stratification leads to biased estimation of the causal effect of exposure on outcome. However, the bias is small relative to the causal relationships between the variables.
  
  
  
  Collider bias can be a partial explanation of the obesity paradox, but unlikely to be the main explanation for a reverse direction of an association to a true causal relationship. Alternative explanations of the obesity paradox should be explored. See Video Abstract at http://links.lww.com/EDE/B51.
authors.affiliation: From the aHealth eResearch Centre, Farr Institute, and bInstitute of Cancer Sciences, Manchester Academic Health Science Centre, University of Manchester, Manchester, United Kingdom.
authors.forename: Matthew
authors.initials: M
authors.lastname: Sperrin
authors.forename: Jane
authors.initials: J
authors.lastname: Candlish
authors.forename: Ellena
authors.initials: E
authors.lastname: Badrick
authors.forename: Andrew
authors.initials: A
authors.lastname: Renehan
authors.forename: Iain
authors.initials: I
authors.lastname: Buchan
mesh_terms: D015982:Bias; D015986:Confounding Factors, Epidemiologic; D006801:Humans; D016015:Logistic Models; D009026:Mortality; D009765:Obesity / Q000453:epidemiology*