[Prognostic factor identified… Now what? What we can and cannot learn from prognostic factor research].

Although it is generally known that a (statistical) association between a factor, i.e., determinant or independent variable, and outcome, i.e., dependent variable, does not directly provide evidence of a causal relation, in practice the distinction between associative and causal relationships often becomes fuzzy when interpreting prognostic factor research. We provide suggestions for interpreting the findings of prognostic factor research. It is important to assess the purpose and design of the study, including the statistical analysis. The actual evidence that prognostic factor research can provide is easily overestimated. In particular when associations between factors and outcome are estimated in a multivariable analysis, causal or predictive qualities can easily but wrongfully be attributed to a prognostic factor. It is generally advisable to refrain from judgments on the causal of predictive qualities of a prognostic factor purely based on a prognostic factor study. Findings from prognostic factor research are usually a good starting point for follow-up research, while the direct applicability of such findings in daily medical practice is often limited.