1. Methodology background


 National Statistic   
 Frequency  Annual
 How compiled  Based on third party data
 Geographic coverage  UK, England and Wales
 Last revised  25 September 2018

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2. About this Quality and Methodology Information report

This quality and methodology report contains information on the quality characteristics of the data (including the European Statistical System five Dimensions of Quality) as well as the methods used to create it.

The information in this report will help you to:

  • understand the strengths and limitations of the data

  • learn about existing uses and users of the data

  • reduce the risk of misusing data

  • help you to decide suitable uses for the data

  • understand the methods used to create the data

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3. Important points

  • Estimates of the very old are mid-year population estimates by single year of age and sex for ages 90 to 104 years and for the 105 years and over age group; estimates are rounded to the nearest 10.

  • Office for National Statistics produces these annually for England, Wales and the UK; comparable statistics for Scotland and Northern Ireland are produced by National Records of Scotland and Northern Ireland Statistics and Research Agency respectively and are used in our UK estimates.

  • For the most accurate data use the most recent version of estimates of the very old, even if you wish to use estimates for a previous year, as the accuracy of the estimates in previous years improves with each successive version.

  • Each version includes estimates going back to 2002.

  • Estimates of the very old are calculated from death registration data using the Kannisto-Thatcher (KT) method; they are constrained to the age 90 years and over totals in the official published mid-year population estimates.

  • Because the KT method relies on death registrations from previous years, it under-estimates in conditions of decreasing mortality. Due to this, estimates of the very old are constrained to the published mid-year population estimates for ages 90 years and over; this means that while the mid-year population estimates determine the total population size, the KT method determines the distribution of the population at different ages.

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4. Quality summary

Overview

Data sources for estimates of the very old are death registration data and official mid-year population estimates of those aged 90 years and over.

The estimates were designated as National Statistics in 2011.

For a particular mid-year (30 June), estimates are published approximately 15 months after the reference date.

Estimates of the very old (including centenarians) provide estimates by sex and single year of age for people aged 90 to 104 years and for the 105 years-and-over age group.

Interest in population estimates at the oldest ages by single year of age has increased as life expectancy has increased and the number of centenarians grows. In recognition of this we began publishing these estimates in 2007.

Estimates are based on death registrations statistics and are constrained to be consistent with the 90 and over totals in the official annual mid-year population estimates. A guide to calculating estimates of the very old is available.

Information on the suitability of the death registration data used in producing these statistics is provided in the quality assurance of deaths data report. Information on the quality of the mid-year population estimates is provided in the mid-year estimates quality and methodology report.

Statistics are produced for the UK, England and Wales separately. Comparable estimates for Scotland and Northern Ireland are produced and published by the National Records of Scotland (NRS) and the Northern Ireland Statistics and Research Agency (NISRA) respectively. These feed into our UK estimates.

These data are produced primarily for our internal customers who require them for the production of national life tables and national population projections. But interest in these statistics has increased and external users of the data include demographers, actuaries, medical researchers and others interested in longevity, population numbers and projected mortality rates.

Uses and users

The estimates of the very old are produced primarily for Office for National Statistics (ONS) internal customers who require them for the production of national life tables and national population projections.

There is also a growing external user demand for the publication of population estimates at the oldest ages by single year of age as life expectancy increases and the very old population grows.

In addition to national and local government users, the estimates of the very old are also used by demographers, actuaries, medical researchers and others interested in longevity, population numbers and/or past and projected age-specific mortality rates at the oldest ages.

We have not conducted a formal user consultation but we have regular contact with our users. User need is evident from requests for these data, for example, from the Department of Health, for use in outputs such as dementia prevalence rates and at both national and local government level for health and social care planning. The estimates also feed into work by the Department for Work and Pensions and HM Treasury in formulating or assessing future policy on pensions.

We identified a user need to have separate Kannisto-Thatcher (KT) estimates for England and Wales and implemented this for the 2016 release. Previously KT estimates were published for England and Wales combined.

Strengths and limitations

Strengths of these statistics are:

  • they are calculated using internationally-recognised methods

  • the calculation uses high-quality administrative data

  • existing data sources are used

  • they are produced annually, providing timely statistics to users

  • they are consistent with the official mid-year estimates of the population aged 90 years and over

  • equivalent comparable estimates are produced for the UK, England and Wales (separately) by ONS and for Scotland and Northern Ireland by NRS and NISRA respectively

Limitations of these statistics are:

  • they are estimates and therefore some error is inevitable

  • they are published rounded to the nearest 10 and grouped at age 105 years and over

  • the method would not produce reliable results at local area geographies because of internal migration and the small numbers involved for the population aged 90 years and over

  • there is a15 months lag between reference and publication dates

Recent improvements

In 2016, we published a report investigating the accuracy of high age population estimates. It shows that although the estimates are of very high quality a small improvement could be made to the KT estimates for England and for Wales if the input deaths data could be acquired directly in the format required by the KT method (that is, deaths by age at the beginning of the reference period) rather than, as is currently done, adjusting deaths by age at death in a calendar year). The feasibility of acquiring input deaths data in the required format is currently being investigated.

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5. Quality characteristics

Relevance

Estimates of the very old provides estimates by sex and single year of age for persons aged 90 to 104 years and for the 105 years and over age group.

Office for National Statistics (ONS) produces these estimates for England, Wales and for the UK. Estimates for Scotland and Northern Ireland are produced and published by the National Records of Scotland (NRS) and the Northern Ireland Statistics and Research Agency (NISRA) respectively. These feed into the UK estimates produced by ONS.

The primary use of these statistics is in the production of national life tables and national population projections. Prior to 2007, these estimates were made available for research purposes but were not officially published. We began to publish these estimates as Experimental Statistics in 2007, in recognition of increased interest in population estimates at the oldest ages. At that time, they were named Estimates of the Very Elderly. Since 2010, we have also published these estimates at UK level.

In 2011, Estimates of the Very Elderly were assessed by the UK Statistics Authority and given National Statistics status. They were published as National Statistics for the first time in September 2011. In 2013, the estimates were renamed as Estimates of the Very Old (including Centenarians).

In 2015, the Estimates of the Very Old (including Centenarians) were reassessed by the UK Statistics Authority. The UK Statistics Authority required ONS to assess the feasibility of producing estimates of the very old for England and Wales separately, rather than combined, as previously published. Separate estimates for England and Wales have been published since 2016.

The estimates are produced annually and published in datasets on the ONS website together with a statistical bulletin providing description and commentary on the estimates.

In addition to their primary use in the production of life tables and population projections, estimates of the very old are used for resource allocation and planning for older people. They are of policy interest because of implications for pensions, health and social care. They are used by:

  • Department for Work and Pensions, the Department of Health and HM Treasury for formulating or assessing future policy on pensions and healthcare

  • demographers, health and medical professionals and other researchers interested in longevity

  • life insurance companies and the actuarial profession for the calculation of mortality rates at the oldest ages

  • answering Parliamentary Questions and responding to media and public interest

Estimates of the very old are constructed using the Kannisto-Thatcher (KT) model of population at advanced age. They are based on death registration statistics and are constrained to be consistent with the 90 years and over totals in the official annual mid-year population estimates. A guide to calculating estimates of the very old is available.

Accuracy and reliability

Estimates of the very old are constrained to the ONS mid-year population estimates of those aged 90 years and over by sex. Mid-year population estimates are produced using a well-established demographic approach called the cohort component method. This involves combining information from a number of data sources including the previous census, survey data and administrative registers. The data sources used are the best that are available on a nationally consistent basis down to local authority level; however, the estimates are subject to the coverage and error associated with these sources.

Any error in the 90 years and over census estimate is carried forward to the inter-censal mid-year population estimates and will be reflected in the estimates of the very old. In addition to non-response, other possible sources of error in the census estimate for people aged 90 years and over include inaccuracies in reporting of dates of birth (for example, proxy reporting by carers). Further information on the quality of the mid-year population estimates, including a brief explanation of the cohort component method can be found in the Quality and Methodology Information report for mid-year population estimates.

Survivor ratio methods such as the KT method provide age-specific estimates of the population for those aged 90 years and over using data from death registrations. The main assumption in these methods is that all deaths are recorded and that the recording of information on age at date of death is sufficiently accurate and reliable. Statistics on death registrations are collected through administrative sources, maintained by the General Register Office (GRO). These data are considered very reliable for two reasons. First, there is a legal requirement to register a death and the certificate issued at registration is needed and used by the recipient. Second, administrative data are not subject to sampling error in the way that survey data are.

The KT method, as used in the production of age 90 years and over estimates for England and for Wales, makes two further assumptions. First, deaths data for England and Wales are published as “age at death”, for a calendar year. In the KT method an assumption is made that these figures are evenly distributed between the two relevant cohorts (years of birth).

The calendar year estimates produced by the method are then interpolated to the mid-year point before being constrained to the official published mid-year population estimate of those aged 90 years and over. In effect this means that, while the KT method determines the estimated distribution of the population aged 90 years and over, the accuracy of the overall KT estimates is dependent on the accuracy of the 90 years and over total in the official mid-year estimates.

Second, it is assumed that international migration at the oldest ages is minimal so this component of population change can be ignored.

The survivorship ratio used is weighted over five years; this takes into account variations in the cohort size at each specific age. Age-specific survivorship ratios are calculated using age-specific deaths data in both the denominator and, in a more complex way, the numerator. Registered deaths are a component of population change and are included in mid-year population estimates. When creating mortality rates for the population aged 90 years and over users should be aware that deaths data have been used to generate the estimates of the very old (the denominator in the calculation).

Estimates of the very old are published rounded to the nearest 10 people.

Output quality

The KT method overestimates the population if their mortality is rising over time and underestimates the population if their mortality is falling. This is because the estimation process takes into account the mortality in previous years, which may not be the same as in the most recent year.

In the recent past, mortality in the UK has typically been falling. In turn, the KT method usually produces lower estimates of the total population aged 90 years and over than the official mid-year estimate. The gap between the KT 90 years and over totals and the official 90 years and over mid-year estimate (MYE) totals will always be largest for the most recent years. This is because there will be higher proportions of people still alive at ages 90 years and over in recent years, meaning the deaths data, used to calculate the KT estimates, are less complete for the most recent years.

Looking at the 2002 to 2014 series, the KT 90 years and over totals were around 98% of the MYE 90 years and over totals for the years 2002 to 2008, but after this point they start to decrease, falling to 88% by 2014 (Figure 1).

The KT method clearly results in an underestimation of the age 90 years and over population for the most recent years. Estimates produced by the method for the current year and the back years are therefore constrained to sum to the 90 years and over totals in the MYEs.

Constraining the KT estimates to sum to the 90 years and over totals in the MYE also provides users with a consistent set of single year estimates beyond age 89 years and produces a smooth join between the population estimates produced by the two methods, cohort component and KT, at the age 89 and 90 years boundary (Figure 2).

There is a trade-off between timeliness and accuracy in the production of estimates of the very old.

Estimates of the very old are produced using death registrations for the reporting year and death occurrences for all preceding years. This is because there is a time lag between the occurrence of a death and registration of that death (particularly for deaths referred to the coroner for investigation) with deaths data by date of occurrence not being available until several months after the availability of death registration data. The number of deaths registered is replaced by the number of deaths occurring in that year when the following year’s estimates are calculated.

Coherence and comparability

We use an internationally recognised methodology in the construction of the estimates of the very old.

We calculate the estimates of the very old for England and Wales and, until 2010, also did so for Northern Ireland. Scotland and Northern Ireland use the same methodology to produce their age 90 years and over estimates by single year of age. The estimates are therefore comparable across UK countries and this allows the estimates to be aggregated to produce estimates for the UK. A report has been published on the comparability of estimates of the very old produced by ONS, NRS and NISRA.

Comparable time series are published back to the year 2002. Each annual set of estimates of the very old is derived using the same methodological approach. A feature of the methodology used is that previous years’ estimates may change when a new year of data is added. Estimates are constrained to the published mid-year population estimate for the 90 years and over age group for the reporting year and re-constrained for previous years as the series is updated. The estimates of the very old are therefore consistent with the mid-year population estimates for England, Wales and the UK.

The estimates are always published as “provisional” because updates are made to the back series of data every time a new year’s figures are added.

Mid-year population estimates for the UK and constituent countries are produced for those aged up to 89 years by single year of age and for age 90 years and over using the cohort component method (see population estimates methodology). The census provides the mid-year resident population base for the cohort component method. Due mainly to levels of uncertainty in the reporting of age in the census at older ages, this method is not used to produce single year of age data for those aged 90 years and over. Estimates of the very old are constrained to the 90 years and over totals in the mid-year population estimates and are therefore consistent with them. However, due to the differences in the methods used to compile these sets of estimates, there may be some discontinuities between the oldest age in the mid-year population estimates (89 years) and the next age (90 years), where the figure is derived by the KT method.

Following the 2011 Census, the mid-year population estimates were revised back to mid-2002. Estimates of the very old were also revised accordingly.

Concepts and definitions (including list of changes to definitions)

These statistics provide estimates of those aged 90 years and over by sex and single year of age up to age 104 years and for the 105 years and over age group for England, Wales and the UK.

Standard classification: National Statistics Country Classification. The National Statistics (NS) Country Classification is based on ISO 3166-1, adapted to meet data needs of UK National Statistics' users and producers.

For the purposes of the NS Country Classification, a country is the name, either short or official, of a current country, dependency or other geographic area of interest. This includes administrative subdivisions, particularly the nations of the UK: England, Northern Ireland, Scotland and Wales.

NRS and NISRA publish equivalent estimates for Scotland and Northern Ireland respectively, using the same definitions, classifications and methodology. These feed into the UK estimates.

There is no legislation governing the output. There is no deviation from agreed standards.

Geography (including list of changes to boundaries)

Estimates of the very old are produced by ONS for England and Wales separately, and for the UK.

Accessibility and clarity

Our recommended format for accessible content is a combination of HTML web pages for narrative, charts and graphs, with data being provided in usable formats such as CSV and Excel. Our website also offers users the option to download the narrative in PDF format. In some instances, other software may be used, or may be available on request. Available formats for content published on our website but not produced by us, or referenced on our website but stored elsewhere, may vary.

For information regarding conditions of access to data, please refer to the following links:

In addition to this Quality and Methodology Information, basic quality information relevant to each release is available in the quality and methodology section of the relevant statistical bulletin: Estimates of the very old (including centenarians) for the UK, 2002 to 2017.

Any enquiries regarding the estimates of the very old can be sent to pop.info@ons.gov.uk.

Timeliness and punctuality

Estimates of the very old (including centenarians) are usually published annually at the end of September. For a particular mid-year (30 June) they become available about 15 months after the reference date. This time lag reflects the availability of the data sources and the time required to process the data and calculate the estimates.

The publication date for the estimates of the very old is determined by the availability of the mid-year population estimates and the death registrations data. Late occurrences of registration data are used to update the previous year’s estimate before compiling the current year’s estimate.

The publication of the estimates of the very old would be later than the planned date only if the input data used to calculate the estimates were not available, for example, if deaths data were unavailable, or if substantial problems were encountered with the processing systems used to calculate the estimates. In previous years the pre-published publication date has always been met. In the year following the release of census estimates, the estimates of the very old are published later than the usual September release date.

For more details on related releases, the GOV.UK Statistics: Release Calendar provides 12 months’ advance notice of release dates. In the unlikely event of a change to the pre-announced release schedule, public attention will be drawn to the change and the reasons for the change will be explained fully at the same time, as set out in the Code of Practice for Statistics.

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6. Methods

How we collect the data, main data sources and accuracy of data sources

Estimates of the very old are calculated from death registration data. They are constrained to age 90 years and over totals in the official published mid-year population estimates.

Statistics on death registrations are collected through administrative sources, maintained by the General Register Office (GRO). These data are considered very reliable for two reasons. First, there is a legal requirement to register a death and the certificate issued at registration is needed and used by the recipient. Second, administrative data are not subject to sampling error in the way that survey data are.

Mid-year population estimates for the UK and constituent countries are produced for those aged up to 89 years by single year of age and for age 90 years and over using the cohort component method (see population estimates methodology).

How we process the data

Estimates of the very old are produced using the Kannisto-Thatcher (KT) method. The KT method is a version of the survivor ratio methodology which produces age-specific estimates of population at older ages (90 years and over) using data from death registrations.

The KT method uses “age at death” data to build up distribution profiles of the numbers of very old people in England and Wales in previous years. For example, if someone dies in 2006 aged 105 years, then this means that they were alive and aged 104 years in 2005 and 103 years in 2004 and so on. By collating “age at death” data for a series of years, it becomes possible to make an estimate of the number of people of a given age alive in any particular year and so create age distribution profiles, assuming that migration at these oldest ages is minimal.

To make estimates for the latest year, it is not possible to use death data, as we are interested in the population who are currently or very recently alive. So for each cohort, the KT method uses an average of the last five years of “age at death” information to estimate the number of survivors for the current year. Each year as more recent deaths data become available to inform the age distribution profiles, estimates for the back years are recalculated and become more accurate. Estimates of the very old (including centenarians) for the UK are produced by aggregating the 90 years and over single year estimates by sex for England, Wales, Scotland and Northern Ireland.

A guide to Calculating estimates of the very old is available.

Further detail on the Demography of Centenarians and the Kannisto-Thatcher method can be found in Thatcher, R (1999)(PDF, 145KB) and Thatcher, R. Kannisto, V. Andreev, K.F. (2002).

A summary of the cohort component method used to produce mid-year population estimates for ages up to 90 years can be found on the population estimates methodology page.

How we quality assure the data

All calculations are replicated using different software packages to check accuracy of the figures.

Visual quality assurance is also carried out; this includes checking that:

  • age and cohort distributions are plausible

  • there are no large year-on-year changes

  • totals sum to the annual mid-year population estimates

How we disseminate the data

The estimates of the very old are published annually. Links from the release calendar make the release date and location of each new set of estimates clear. A statistical bulletin, which describes the main patterns and trends in the data accompanies the estimates. The estimates and the underlying data for the charts and tables in the bulletin can be downloaded free of charge in Microsoft Excel format.

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Contact details for this Methodology

Ngaire Coombs
pop.info@ons.gov.uk
Telephone: +44 (0)1329 444661