Objectives Uncertainties exist about when and how best to adjust performance measures for case mix. Our aims are to quantify the impact of case-mix adjustment on practice-level scores in a national survey of patient experience, to identify why and when it may be useful to adjust for case mix, and to discuss unresolved policy issues regarding the use of case-mix adjustment in performance measurement in health care.
Design/setting Secondary analysis of the 2009 English General Practice Patient Survey. Responses from 2 163 456 patients registered with 8267 primary care practices. Linear mixed effects models were used with practice included as a random effect and five case-mix variables (gender, age, race/ethnicity, deprivation, and self-reported health) as fixed effects.
Main outcome measures Primary outcome was the impact of case-mix adjustment on practice-level means (adjusted minus unadjusted) and changes in practice percentile ranks for questions measuring patient experience in three domains of primary care: access; interpersonal care; anticipatory care planning, and overall satisfaction with primary care services.
Results Depending on the survey measure selected, case-mix adjustment changed the rank of between 0.4% and 29.8% of practices by more than 10 percentile points. Adjusting for case-mix resulted in large increases in score for a small number of practices and small decreases in score for a larger number of practices. Practices with younger patients, more ethnic minority patients and patients living in more socio-economically deprived areas were more likely to gain from case-mix adjustment. Age and race/ethnicity were the most influential adjustors.
Conclusions While its effect is modest for most practices, case-mix adjustment corrects significant underestimation of scores for a small proportion of practices serving vulnerable patients and may reduce the risk that providers would ‘cream-skim’ by not enrolling patients from vulnerable socio-demographic groups.
- General practice
- family medicine
- quality of care
- healthcare quality improvement
- primary care
- health services research
- quality measurement
- patient satisfaction
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Funding The study was funded by a grant from the UK Department of Health. The views expressed are those of the authors and not of the Department of Health.
Competing interests The authors have no conflicts of interest as defined by the International Committee of Medical Journal Editors. MR and JC act as academic advisors to Ipsos MORI for the development of the General Practice Patient Survey and have received remuneration for this.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement Individual level data were provided to us by Ipsos MORI with a covering confidentiality agreement with the Department of Health. Under this agreement we are not at liberty to share the dataset with third parties.