Nursing homes’ publicly reported star ratings have increased substantially since the adoption of the Five-Star Quality Rating System. But there hasn’t been a corresponding drop in preventable hospitalizations, according to a new study, and “teaching to the test” could be a culprit.

That’s the result of a new study from the University of Pennsylvania and University of Chicago, published in the Journal of Health Care Organization, Provision and Financing.

Poring over Medicare claims data from 2007 to 2010, investigators aimed to track the association between nursing homes’ ratings and the number of potentially preventable hospitalizations per 30 days.

Researchers found that, before the Five-Star system began in 2008, there was a consistent relationship between star rating and hospitalization rates, with the higher-quality facilities demonstrating lower numbers of readmits. After the release in December 2008, however, the gradient between star rating and hospitalizations reduced.

“This aligns with expectations that the ratings are becoming less meaningful over time as a broad indicator of quality,” researchers wrote.