Developing a video-based method to compare and adjust examiner effects in fully nested OSCEs

Abstract

Background: Although averaging across multiple examiners' judgements reduces unwanted overall score variability in objective structured clinical examinations (OSCE), designs involving several parallel circuits of the OSCE require that different examiner cohorts collectively judge performances to the same standard in order to avoid bias. Prior research suggests the potential for important examiner-cohort effects in distributed or national examinations that could compromise fairness or patient safety, but despite their importance, these effects are rarely investigated because fully nested assessment designs make them very difficult to study. We describe initial use of a new method to measure and adjust for examiner-cohort effects on students' scores.

Methods: We developed video-based examiner score comparison and adjustment (VESCA): volunteer students were filmed 'live' on 10 out of 12 OSCE stations. Following the examination, examiners additionally scored station-specific common-comparator videos, producing partial crossing between examiner cohorts. Many-facet Rasch modelling and linear mixed modelling were used to estimate and adjust for examiner-cohort effects on students' scores.

Results: After accounting for students' ability, examiner cohorts differed substantially in their stringency or leniency (maximal global score difference of 0.47 out of 7.0 [Cohen's d = 0.96]; maximal total percentage score difference of 5.7% [Cohen's d = 1.06] for the same student ability by different examiner cohorts). Corresponding adjustment of students' global and total percentage scores altered the theoretical classification of 6.0% of students for both measures (either pass to fail or fail to pass), whereas 8.6-9.5% students' scores were altered by at least 0.5 standard deviations of student ability.

Conclusions: Despite typical reliability, the examiner cohort that students encountered had a potentially important influence on their score, emphasising the need for adequate sampling and examiner training. Development and validation of VESCA may offer a means to measure and adjust for potential systematic differences in scoring patterns that could exist between locations in distributed or national OSCE examinations, thereby ensuring equivalence and fairness.

Keywords: OSCEs; assessment; assessor variability; psychometrics.

Figure 1

Bland–Altman plots of global score (left) and total percentage score (right); positive difference indicates live scored higher than video. Bold dotted lines represent the mean difference and 95% confidence intervals [CIs] (limits of agreement) of the differences, and the light dotted lines represent the 95% CIs for these values

Figure 2

Wright map showing relative influence of students, stations and examiner cohorts on overall global scores

Figure 3

Diagram showing the relative influence of students, stations and examiner cohorts on total percentage scores

Figure 4

Plot of raw versus model‐adjusted overall global scores for individual students for global and total percentage scores. Note: students indicated by triangles (▴) changed from fail to pass when scores were adjusted; students indicated by diamonds (♦) changed from pass to fail when scores were adjusted; students indicated by squares (■) passed under both conditions; and students indicated by circles (●) failed under both conditions