CMS explains the new 2021 methodology for its Hospital Quality Star Ratings uses “a simple average of measure scores to calculate measure group scores and Z-score standardization to standardize measure group scores” in five measure groups.1

The measure groups are safety of care (weighted at 22%), mortality (22%), patient experience (22%), readmission (22%), and timely and effective care (12%). CMS uses these groups to calculate a weighted average. The agency combines group scores into one facility summary score. When a hospital is missing a measure group, CMS redistributes weights proportionally among the other groups.

Then, facilities are assigned to one of three peer groups. CMS determines this assignment by how many measure groups the hospital reported (three, four, or five). Leaders must report at least three measures within three measure groups to qualify for a star rating.

“Finally, hospitals are assigned to star ratings within each peer group using k-means clustering so that summary scores in one star rating category are more similar to each other and more different than summary scores in other star rating categories,” CMS explains.1 


  1. CMS. Inpatient public reporting preview help guide. January 2021 public reporting preview/April 2021 Care Compare release.