Linkage methodology

The ONS mortality data is linked to HES by matching person identifiable data in the ONS mortality data with patient identifiers in HES. The linkage process results in assigning a unique patient identifier (PERSON_ID) to the ONS death record. The Master Person Service (MPS) Person ID is present in all HES data sets (Accident and Emergency, Admitted Patient Care and Outpatients) enabling patients to be tracked in a confidential way. More information on the MPS Person ID and its methodology is available. 

The latest processed HES data is always used for linkage. It should be noted that only ONS records that can successfully match to a patient in HES are included in the linked data. ONS records that cannot be matched are rejected and given an opportunity to match every subsequent month when the latest death registrations from ONS are available. An ONS mortality record can match to a record in HES based on 2 different criteria (refer to Matching ONS data to HES). If an ONS record matches to more than 1 PERSON_ID, the best quality match is selected.

The ONS records that have been linked to HES are later merged (refer to the section Merging ONS and HES mortality data) with death records from the HES admitted patient care (APC) data. These records are identified by the hospital discharge method ‘dead’. This is done to ensure that death records of all patients who were discharged dead from hospital and not available in the ONS data or that could not be linked to the ONS data are made available in the linked mortality data.

Figure 1 below is a high-level illustration of the linkage process.

HES captures all activity in English hospitals, but the ONS mortality data contains all deaths registered in England and Wales.  Therefore, when the data is linked it is possible to capture Welsh residents treated in England. The deaths of Welsh residents who haven’t been treated by an English hospital provider will remain unmatched. Poor quality person identifiers in either HES or ONS can also cause the linkage to fail. These 2 reasons account for the majority of unmatched ONS records.

The latest data from both HES and ONS is always used for linkage. HES data is always cumulative for the current financial year. This can cause some activity records to change or disappear when providers of healthcare modify or remove submitted data from a previous month of the same financial year. Further, an annual refresh of the provisional HES data is produced to provide a finalised version of the provisional monthly data.

The linkage methodology uses ONS mortality data based on registered deaths.  Registered deaths are deaths that were registered in a period as opposed to death occurrences within that period. Registered deaths are available on a weekly basis and therefore are timelier compared to death occurrences. Sometimes the year in which a death is registered may not be the same as the year in which a death occurred such as when a coroner’s investigation is required for example if a death is suspicious. Until a coroner’s investigation is completed, a death cannot be registered.

ONS publishes mortality data based on the calendar year of death registration. Just as in HES, ONS provisional data has not been subject to full quality assurance and may not contain all deaths which were registered, or which occurred during the period.  Deaths which occurred in a given year may be registered in a subsequent year.  The linkage process remains open to accept these registrations, so figures are subject to change.

Further, an annual refresh of ONS data is received each year covering the most recent calendar year, to provide a finalised position on the provisional data. Further information on ONS mortality data is available.

Each month a query is run against the latest cumulative HES APC data to obtain deaths in hospitals.  This uses HES APC data from 1997-98 onwards. In the unlikely event that the same PERSON_ID is included in more than 1 death record, all but 1 of the records with the same PERSON_ID are removed. The following rules are applied sequentially to determine which records are deleted:

1. Delete all records sharing the same PERSON_ID except the one with the latest financial year [FYEAR].

2. If duplicates still remain, delete all records sharing the same PERSON_ID except the one with the latest discharge date [DISDATE].

3. If duplicates still remain, delete all records sharing the same PERSON_ID except the one with the latest episode end date [EPIEND].

4. If duplicates still remain, delete all records sharing the same PERSON_ID except the one with the latest episode key [EPIKEY].

The mortality data received from ONS is matched with patient data in HES. As a result, most ONS death records are assigned a unique patient identifier PERSON_ID, while the others are rejected. It is to be noted that the ONS data for a particular month is available before HES data for the same month.  Every subsequent month the rejected ONS records get a chance to match again with the HES data. This ensures that the unmatched ONS records are not permanently rejected.  It also allows any HES records that have changed in subsequent monthly submissions another chance to link to ONS records.

The mortality data received from ONS have undergone a Personal Demographics Service (PDS) matching process which produces a confirmed NHS number. Mostly, this is done via an automated process, but manual tracing is carried out if the automated process cannot assign and NHS number to the record. Approximately 99% of records in the ONS mortality data have a confirmed NHS number. Any ONS records with an invalid date of death are excluded from the linkage process. Only current, non-cancelled records are included to make sure that latest records for each individual are used (for example if there have been updates to a particular record since it was first included in the data set).

All of the records in HES have gone through the Master Patient Service (MPS) which creates a field called PERSON_ID.  This contains the NHS number if it has been found. If the NHS number cannot be found, then a PERSON_ID is created for the record.  For the vast majority of records in the HES APC data, the PERSON_ID is the same as the NHS number.

The HES and ONS data are then linked using the different matching methods described below. The match rank obtained from the HES-ONS linkage process is an indication of the level of confidence that an ONS death record has been correctly matched to a patient in HES. The lowest rank (1) is considered the best quality match and the higher match rank (8) the lowest quality match.  A match rank of 0 means that no match was made.

The match ranks used are as follows¹:

• MATCH_RANK = 0 indicates that the death record is present in HES only, because an ONS record could not be matched to HES, or the death record is not available in the ONS data
• MATCH_RANK = 1 corresponds to an exact match between the confirmed NHS number in the ONS data and the PERSON_ID in the HES data
• MATCH_RANK = 8 corresponds to an exact match of date of birth, sex and postcode for the ONS and HES data (excluding any records where the date of birth is 1 January)

Depending on the steps used during the matching, it is possible for an ONS mortality record to match to more than 1 PERSON_ID.  The record with the best match rank is always used in the linked data. It is highly unlikely that all matches have the same match rank. In the absence of a definitive way of deciding which match is best, the inconsistency is tolerated and a single PERSON_ID is applied to each death record.

Figure 2 Percentage of records corresponding to each match rank in the HES-ONS linked data for April 2023

This is obtained by linking the mortality data in ONS up to February 2023 with the HES data until January 2023.