A statistical correction that accounts for how sick a hospital's patients were on arrival, so outcomes can be compared between hospitals treating different populations.
Why this matters to you
Without it, every hospital that takes complex cases would look dangerous and every hospital that treats mild cases would look excellent. With it, the published figure estimates the hospital's own contribution. The correction is good but incomplete: it uses age, sex and documented diagnoses, and cannot see income, housing, language, transport, or social support.
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.