Generalisability in unbalanced, uncrossed and fully nested studies.

OBJECTIVES There is growing interest in multi-source, multi-level feedback for measuring the performance of health care professionals. However, data are often unbalanced (e.g. there are different numbers of raters for each doctor), uncrossed (e.g. raters rate the doctor on only one occasion) and fully nested (e.g. raters for a doctor are unique to that doctor). Estimating the true score variance among doctors under these circumstances is proving a challenge. METHODS Extensions to reliability and generalisability (G) formulae are introduced to handle unbalanced, uncrossed and fully nested data to produce coefficients that take into account variances among raters, ratees and questionnaire items at different levels of analysis. Decision (D) formulae are developed to handle predictions of minimum numbers of raters for unbalanced studies. An artificial dataset and two real-world datasets consisting of colleague and patient evaluations of doctors are analysed to demonstrate the feasibility and relevance of the formulae. Another independent dataset is used for validating D predictions of G coefficients for varying numbers of raters against actual G coefficients. A combined G coefficient formula is introduced for estimating multi-sourced reliability. RESULTS The results from the formulae indicate that it is possible to estimate reliability and generalisability in unbalanced, fully nested and uncrossed studies, and to identify extraneous variance that can be removed to estimate true score variance among doctors. The validation results show that it is possible to predict the minimum numbers of raters even if the study is unbalanced. DISCUSSION Calculating G and D coefficients for psychometric data based on feedback on doctor performance is possible even when the data are unbalanced, uncrossed and fully nested, provided that: (i) variances are separated at the rater and ratee levels, and (ii) the average number of raters per ratee is used in calculations for deriving these coefficients.