OBJECTIVE: The objective of this study was to identify modifiable factors that improve the reliability of ratings of severity of health care-associated harm in clinical practice improvement and research. METHODS: A diverse group of clinicians rated 8 types of adverse events: blood product, device or medical/surgical supply, fall, health care-associated infection, medication, perinatal, pressure ulcer, surgery. We used a generalizability theory framework to estimate the impact of number of raters, rater experience, and rater provider type on reliability. RESULTS: Pharmacists were slightly more precise and consistent in their ratings than either physicians or nurses. For example, to achieve high reliability of 0.83, 3 physicians could be replaced by 2 pharmacists without loss in precision of measurement. If only 1 rater was available for rating, approximately 5% of the reviews for severe harm would have been incorrectly categorized. Reliability was greatly improved with 2 reviewers. CONCLUSIONS: We identified factors that influence the reliability of clinician reviews of health care-associated harm. Our novel use of generalizability analyses improved our understanding of how differences affect reliability. This approach was useful in optimizing resource utilization when selecting raters to assess harm and may have similar applications in other settings in health care.