How many infections a hospital actually had, divided by how many would be expected given its patient mix, device use and procedures. 1.00 means as many as expected.
Why this matters to you
The adjustment is what makes the comparison fair. A large teaching hospital running hundreds of ICU central lines will record more infections in raw numbers than a small community hospital, and comparing counts would punish it for treating sicker patients. Below 1.00 is better than expected; 0.50 means half as many infections as predicted. Hospitals with few eligible cases have results suppressed, which is why small hospitals often show nothing here.
Official source
The authoritative definition and current rules are published here:
Rules and figures change. Where this page and the official source disagree, the official source is correct — and we would like to know, so please tell us.