11. Quality and methodology

Deaths data sources

A provisional extract of death registrations and death occurrences data for Quarter 4 (1 October to 31 December) 2017 was created on 29 January 2018, roughly four weeks after the end of the reporting period. For this reason, we would expect death occurrences to increase, because of registration delays, which will not be accounted for by 29 January 2018. In exceptional circumstances there may also be changes to the number of registrations but these would be very small. Registrations data for years prior to 2017 are final, whereas occurrences data prior to 2016 are final.

Registration delays on occurrences

In England, deaths should be registered within five days of the death occurring, but there are some circumstances that result in the registration of the death being delayed. Deaths considered unexpected, accidental or suspicious will be referred to a coroner who may order a post-mortem or carry out a full inquest to ascertain the reasons for the death. The coroner can only register the death once any investigation is concluded and they are satisfied that the death was natural and that the cause of death has been certified correctly.

If the coroner is not satisfied that the death was from natural causes then an inquest will normally be held to determine the cause of death. The time taken to investigate the circumstances of the death can often result in a death registration exceeding the five-day grace period and these are defined as registration delays. While delays are commonly only a few days, registration delays can extend into years, particularly for deaths from external causes when inquests are held. We are only aware of a death and able to include it in the statistics once it has been registered.

Those at younger ages are disproportionally affected by registration delays due to external causes of death being more common in these ages. However, in general, deaths at such ages are not very common and make up only a small percentage of all deaths.

The death occurrences dataset for 2017 will not hold all deaths that occurred in the quarter due to late registrations. Where death occurrences have been used in this report, deaths for previous years have been extracted using a similar extraction date as the 2017 occurrences data. This allows for control over registration delays.

Expected deaths methodology

For each respective year, single year of age mortality rates were calculated. These were then applied to the population projections for 2017 to calculate the number of expected deaths in each single year of age using the mortality rate from the respective year. From this we were able to calculate the difference between observed and expected deaths in 2017.

Quarterly population denominators

We publish the mid-year population estimates used for calculating rates. For 2017, the 2016-based ONS population projections were used.

Single year of age populations for the oldest ages (90 to 100 years and over) for 2002 to 2016 were taken from the mid-year population estimates of the very old publication. For 2001, the population estimates for ages 90 years and over were used and for 2017 the 2016-based ONS population projections were used.

Calculation of mortality rates for quarterly deaths requires adjustments to be made to annual population estimates in order to calculate rates that are comparable with annual rates.

We calculate an annual population centred on the mid-point of the quarter using two years’ worth of population estimates or projections. This is then multiplied by the proportion of the number of days within a quarter of the total number of days within that year. The output is used as the population denominator in calculations of age-standardised and age-specific morality rates.

Where m is the number of days from 1 July 2016 (the start of the mid-year for the population estimate) to the midpoint of the relevant quarter, inclusive, N is the number of days in the Quarter for example Quarter 4 (Oct to Dec) 2017 and M is the number of days in 2017 and (i) is the age group.

This method is very similar to that used to calculate population denominators for quarterly conception rates.

Comparability ratios for causes of death

We code cause of death using the World Health Organisation's (WHO) International Classification of Diseases, Tenth Revision (ICD-10). Where possible, deaths are automatically coded using specialist software, with the remaining deaths being manually coded.

ICD-10 was introduced in England and Wales in January 2001. Since then various amendments to the ICD-10 have been authorised by WHO and we have updated cause coding software to incorporate these changes. Between 2001 and 2010, we used software version 2001.2; between 2011 and 2013, version 2010 was used and on 1 January 2014, we changed the software to a package called IRIS (version 2013). IRIS software version 2013 incorporates all official updates to ICD-10 approved by WHO, which were timetabled for implementation before 2014.

To understand the impact of these changes on mortality statistics, we carried out bridge coding studies in 2011 and 2014 in which samples of deaths that had previously been coded using the old software were then independently recoded using the new version of ICD-10. This dual-coded data can be used to produce comparability ratios that can be applied to data to account for the changes to allow more consistent time series to be produced. We do not ordinarily publish data that have had comparability ratios applied, but for the purposes of this article, comparability ratios for deaths in people aged 75 years and over have been calculated and applied. The full set of ratios and their confidence intervals can be found in the accompanying datasets.

The Mortality Quality and Methodology Information report contains important information on:

the strengths and limitations of the data and how it compares with related data

uses and users of the data

how the output was created

the quality of the output including the accuracy of the data

The User guide to mortality statistics is also a useful resource when reporting mortality statistics.