colliderbias

Collider bias, also known as selection bias or Berkson's bias, refers to a systematic error that occurs in observational studies when the process of selecting participants causes a distortion in the estimated relationships between variables. This bias typically arises when the sample is conditioned on a common effect of the exposure and outcome, such as a specific health status or a particular characteristic, leading to spurious associations or masking true associations.

The phenomenon is especially relevant in epidemiology and clinical research, where it can distort the apparent relationship between risk factors and diseases. For example, selecting participants from a hospital may induce collider bias if hospitalization is influenced by both the exposure and the disease being studied, thus creating artificial correlations or obscuring real ones.

Collider bias differs from confounding because it is not caused by a third variable that influences both the exposure and outcome, but by the selection process itself. When the study design involves conditioning on a variable that is a consequence of both exposure and outcome, the observed associations in the data may not reflect the true causal relationships.

To mitigate collider bias, researchers should carefully consider the study design and avoid conditioning on variables influenced by both exposure and outcome. Analytical methods such as directed acyclic graphs (DAGs) can help identify potential collider variables and guide appropriate adjustments.

Understanding collider bias is critical for accurate causal inference and for reducing misleading conclusions in observational research, ensuring that findings better reflect true biological or social relationships.