PT - JOURNAL ARTICLE AU - Lorenzo Richiardi AU - Neil Pearce AU - Eva Pagano AU - Daniela Di Cuonzo AU - Daniela Zugna AU - Costanza Pizzi TI - Baseline selection on a collider: a ubiquitous mechanism occurring in both representative and selected cohort studies AID - 10.1136/jech-2018-211829 DP - 2019 May 01 TA - Journal of Epidemiology and Community Health PG - 475--480 VI - 73 IP - 5 4099 - http://jech.bmj.com/content/73/5/475.short 4100 - http://jech.bmj.com/content/73/5/475.full SO - J Epidemiol Community Health2019 May 01; 73 AB - There is debate as to whether cohort studies are valid when they are based on a source population that is non-representative of a given general population. This baseline selection may introduce collider bias if the exposure of interest and some other outcome risk factors affect the probability of being in the source population, thus altering the associations between the exposure and those risk factors. We argue that this mechanism is not specific to ‘selected cohorts’ and also occurs in ‘representative cohorts’ due to the selection processes that occur in any population. These selection processes are for example linked to the life status, immigration and emigration, which, in turn, may be affected by environmental and social determinants, lifestyles and genetics. We provide real-world examples of this phenomenon using data on the population of the Piedmont region, Italy. In addition to well-recognised mechanisms, such as shared common causes, the associations between the exposure of interest and the risk factors for the outcome of interest in any source population are potentially shaped by collider bias due to the underlying selection processes. We conclude that, when conducting a cohort study, different source populations, whether ‘selected’ or ‘representative’, may lead to different exposure–outcome risk factor associations, and thus different degrees of lack of exchangeability, but that one approach is not inherently more or less biased than the other. The key issue is whether the relevant risk factors can be identified and controlled.