A Tutorial on Interpreting Bifactor Model Scores

This tutorial addresses possible sources of confusion in interpreting trait scores from the bifactor model. The bifactor model may be used when subscores are desired, either for formative feedback on an achievement test or for theoretically different constructs on a psychological test. The bifactor model is often chosen because it requires fewer computational resources than other models for subscores. The bifactor model yields a score on the general or primary trait measured by the test overall, as well as specific or secondary traits measured by the subscales. Interpreting the general trait score is straight-forward, but the specific traits must be interpreted as residuals relative to the general trait. Trait scores on the specific factors are contrasted with trait scores on a simple-structure model with correlated factors, using example data from one TIMSS test booklet and a civic responsibility measure. The correlated factors model was used for contrast because its scores correspond to a more intuitive interpretation of subscores, and thus it helps to illustrate how the bifactor scores should NOT be interpreted. Estimation details are covered in an appendix.