Introduction

Differential rater function over time (DRIFT) and contrast effects (examiners’ scores biased away from the standard of preceding performances) both challenge the fairness of scoring in Objective Structured Clinical Exams (OSCEs). This is important as, under some circumstances, these effects could alter whether some candidates pass or fail assessments. Benefitting from experimental control, this study investigated the causality, operation and interaction of both effects simultaneously for the first time in an OSCE setting.

Methods

We used secondary analysis of data from an OSCE in which examiners scored embedded videos of student performances interspersed between live students. Embedded video position varied between examiners (early vs late) whilst the standard of preceding performances naturally varied (previous high or low). We examined linear relationships suggestive of DRIFT and contrast effects in all within-OSCE data before comparing the influence and interaction of “Early” vs “Late” and “PreviousHigh” vs “PreviousLow” conditions on embedded video scores.

Results

Linear relationships data did not support the presence of DRIFT or contrast effects. Embedded videos were scored higher early (19.9(19.4-20.5)) vs late (18.6(18.1-19.1), p<0.001) but scores did not differ between PreviousHigh and PreviousLow conditions. The interaction term was non-significant.

Conclusions

In this instance, the small DRIFT effect we observed on embedded video can be causally attributed to examiner behaviour. Contrast effects appear less ubiquitous than some prior research suggests. Possible mediators of these finding include: OSCE context, detail of task specification, examiners’ cognitive load and the distribution of learners’ ability. As the operation of these effects appears to vary across contexts, further research is needed to determine the prevalence and mechanisms of contrast and DRIFT effects, so that assessments may be designed in ways which are likely to avoid their occurrence. Quality assurance should monitor for these contextually variable effects in order to ensure OSCE equivalence.