How many patients are needed to provide reliable evaluations of individual clinicians?

The purpose of this study was to determine how many patients are needed to provide reliable patient ratings of care at the individual clinician level. SETTING AND SOURCES OF DATA: The study was conducted in an academic medical center and was based on analysis of 34,985 patients who completed a 50-item survey rating the care received during a recent outpatient visit to a physician or midlevel provider.

Analyses of patient satisfaction surveys was done to: 1) confirm the dimensions of satisfaction with outpatient care in an existing measure, and 2) determine the number of patients required to provide reliable estimates of clinician care for single items and an 11-item composite scale.

Factor analysis showed that the survey measured 2 dimensions of satisfaction: 1) clinician care, and 2) features of visiting the office. The 11-item clinician care scale had high reliability (Cronbach's alpha=0.97). The number of patients needed to achieve reliability of 0.80 at the clinician level was 66 for the 11-item scale and ranged from 52 to 91 for individual items. For primary care physicians only, the comparable number of patients per clinician was 77 for the 11-item scale and ranged from 50 to 147 across items.

For the survey items that we analyzed, the answer to the question "How many patients are needed to obtain useful and reliable feedback?" is at least 50, but varies by item type (global vs. specific) and by number of items (composite scale or single-item rating) and by the conditions of use (for self-assessment and learning or reward and punishment).