What is it?

Selection [bias] is a [systematic error] in a study that occurs from the process used to identify (select) the study participants, allocate them to study groups and from factors that influence study participation [1][2].

It leads to preferential selection or participation of subjects into the study according to their exposure status or outcome/disease status, with resulting systematic differences in the characteristics of participants between the study and control groups, i.e. the groups differ from each other by factors which may affect the outcome of the study [2]. The measurement of the [association] between exposure and outcome will then differ between those who are included in the study and those who were eligible but not included.

Selection bias may be due to:

• Sampling bias
• Ascertainment bias
  • case ascertainment (surveillance) bias
  • referral/admission bias
  • diagnostic bias
• Participation bias
  • self-selection (volunteerism)
  • healthy worker effect
  • non-response / refusal bias
  • survival bias
  • loss to follow-up

Selection biases in case-control studies] include, among others: case ascertainment (surveillance) bias, referral bias, diagnostic bias, non-response bias, and survival bias.

Selection biases in cohort studies include the healthy worker effect, diagnostic bias, non-response bias, and loss-to-follow-up.

In epidemiological studies, all efforts should be made to avoid biasing the selection of study participants. It is possible to minimize selection bias by paying attention to several factors.

References

1. Rothman KJ. Epidemiology - An Introduction. New York: Oxford University Press; 2002.

2. Bailey L, Vardulaki K, Langham J, Chandramohan D. Introduction to Epidemiology. Black N, Raine R, editors. London: Open University Press in collaboration with LSHTM; 2006.