Verstoring in observationeel onderzoek: ‘confounding’

Conflict of interest: none declared. Financial support: none declared.

• Confounding is an important and common issue in observational (non-randomized) research on the effects of pharmaceuticals or exposure to etiologic factors (determinants).

• Confounding is present when a third factor, related to both the determinant and the outcome, distorts the causal relation between these two.

• There are different methods to control for confounding. The most commonly used are restriction, stratification, multivariable regression models, and propensity score methods.

• With these methods it is only possible to control for variables for which data is known: measured confounders.

• Research in the area of confounding is currently directed at the incorporation of external knowledge on unmeasured confounders, the evaluation of instrumental variables, and the impact of time-dependent confounding.