11. Quality and methodology

The Health state life expectancies 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

What are health state life expectancies?

Health state life expectancies add a quality of life dimension to estimates of life expectancy (LE) by dividing expected life span into time spent in different states of health or disability. Healthy life expectancy (HLE), which estimates lifetime spent in “Very good” or “Good” health, is based on how individuals perceive their general health. The second is disability-free life expectancy (DFLE), which estimates lifetime free from a limiting persistent illness or disability. This is based upon a self-rated assessment of how health conditions and illnesses limit an individual’s ability to carry out day-to-day activities.

Both health state life expectancies are summary measures of population health and important indicators of the well-being of society, providing context to the effects of policy changes and interventions at both national and local levels. Health state life expectancies are used across public, private and voluntary sectors, in the assessment of healthy ageing, fitness for work, monitoring health improvement, fairness in extensions to the State Pension age, pension provision and health and social care need.

Estimates provided in this release provide a snapshot of health status of populations in the UK, assuming mortality and health status observed in 2014 to 2016 remained constant over time. However, health status and mortality rates in national and local area populations may change in the future, due to changes in exposure to health risks and innovations in treatment of diseases affecting health. Therefore, the estimates reported in this bulletin are a guide to the number of years a specific area’s population will live in different health states.

The questions classifying individuals to activity limitation were changed in April 2013 following a review into how disability should be measured in national surveys. Primary harmonised standards for disability data collection and the production of statistical measures of disability (PDF, 136.03KB) are available.

More information about the question change can be seen in the QMI report. Due to this change and the change in the introductory paragraph of the disability section being re-worded in 2010, there has been a difference in the number of people reporting disabilities. Therefore, estimates either side of the discontinuity (before and after April 2013) should not be directly compared.

Changes in health state life expectancies over time can be assessed by comparing non-overlapping time periods. Therefore, estimates for 2014 to 2016 should not be compared with estimates for 2013 to 2015 or 2012 to 2014 for example, as they will contain some of the same survey respondents.

How do we calculate health state life expectancies?

In this bulletin, data were calculated using abridged life table methods (based on five-year age groups). Life expectancy (LE) has been calculated using the standard Chiang II abridged life table method. However, estimation of the variance, used to measure the statistical precision of life expectancy estimates, has undergone one important modification as the Chiang II method assumes zero variance for the final age group. To enable the calculation of a confidence interval for the final age group, the method developed by Silcocks and others (2001) has been used and further information is available in the methods paper published in 2016.

Figures for England, Wales, English regions, counties and local authorities exclude deaths of non-residents. However, Scotland includes non-usual residents who die in Scotland and do not have an area of residence within Scotland and imputation is used to assign the death to an area of “residence” (see Section 1.3.1 in the Healthy life expectancy: technical paper for more detail).

Northern Ireland also includes non-usual residents that are allocated to the place of death (see Life expectancy for Northern Ireland information paper and methodology guide for more detail).

The data used in calculating the general health and disability status were obtained from the Annual Population Survey (APS). They were aggregated over a three-year period to achieve sufficiently large sample sizes to enable meaningful statistical comparison at national and subnational level.

The prevalence of “Good” general health and disability-free among males and females resident in private households in England was compared across areas. HLE was then calculated using the Sullivan method, which combines prevalence data with mortality and mid-year population estimates (MYE) over the same period and geographical coverage to calculate estimates of life expectancy (LE), healthy life expectancy (HLE) and disability-free life expectancy by age and sex (for more information, see the ONS life table template).

The APS provides prevalence information for those aged 16 and over. We are able to estimate lower age groups by using a census imputation method.

Results are presented with 95% confidence intervals in reference tables to help interpretation. Confidence intervals in this statistical bulletin indicate the uncertainty surrounding health state life expectancy estimates and allow more meaningful comparisons between areas. Within this bulletin, a difference that is described as “significant” is statistically significant and has been assessed using the z-test. More information about this z-test can be viewed in Appendix 1 of the Sullivan guide.

What is the difference between the mean, median and modal age at death?

Average life spans are usually measured using the life table. A life table is a standard method of analysing mortality by age to summarise a population’s mortality experiences. These mortality experiences can be presented using three measures of average life span:

the mean

the median

the mode

Median and modal ages at death can be measured using period life tables as well. Period life tables use the current age-specific mortality rates with no assumption about the projected death rates. Therefore, age at death reported here for the given time period is the expected age at death if a person experienced those age-specific mortality rates throughout his or her life.

All three measures of life span are summary measures of ageing in a population, helping to understand mortality improvements over time. They feed into the policies surrounding pension provision and needs in the areas of housing and health and social care.

The mean

The most commonly used measure of average life span is life expectancy at birth or the mean age at death. It is the average number of years a person is expected to live before his or her death. We regularly produce period life expectancy, which is widely understood and used as a robust measure of population health.

The median

The second measure is the median age at death. This estimates the age at which half of a hypothetical cohort would have died and half would still be alive.

The mode

A third measure of life span is the late modal age at death. This measure, which has emerged as an alternative measure of life span in low infant mortality countries such as UK, identifies the age at which the highest number of deaths occur over the life course. This focuses on deaths occurring at older ages in the life table in its calculation.

How it is calculated

Mean age at death (life expectancy at birth) is calculated using the standard Chiang II methods for the abridged life table, where the final age band was closed at age 95 and over to measure the survival at older ages.

In the abridged life table, median and modal age groups are identified using the dx column in the life table, which represents the expected number of deaths in each age group based on the probability of dying in the specific age interval in a hypothetical cohort of 100,000 persons.

By using dx, the modal age group is where the highest number of deaths occurred out of this cohort of 100,000; the median age group is where cumulative deaths (that is, dxi plus dx(i-1)) exceed 50,000. After identifying the median and modal age groups, group data formulas are applied to obtain approximate median and modal ages at death.