The HES-ONS linked mortality dataset combines Hospital Episode Statistics (HES), which contains clinical information on patients' hospital activity and treatment such as diagnoses and procedures, with mortality records from the Office for National Statistics (ONS). The linkage captures deaths of people who have been treated in English hospitals, irrespective of whether they died in hospital or not.

While HES data alone can be used to identify if a patient died in hospital and analysed on the basis of their primary diagnosis, it cannot determine the cause of death or capture deaths after discharge. The ONS mortality data is a richer source of information on deaths, containing cause of death, date and place of death for all deaths registered in England and Wales, based on information from death certificates.

By linking the two datasets, it is possible to analyse deaths both in and outside hospital, assess outcomes such as postoperative mortality (e.g. deaths within 30, 60 or 90 days), track provider performance, and conduct long-term follow-up on survival rates. The dataset only includes mortality information for individuals who have had a record in HES.

SAHSU holds a data sharing agreement with NHS England and has access to a restricted subset of variables from the full dataset.

The dataset may be internally linkable with SAHSU's environmental datasets, including air pollution and green space.