Life expectancy says how long; healthy life expectancy tries to say how well. It is the number behind the question of whether added years are good years, and it is measured in enough different ways that comparing two figures is often comparing two definitions.
The idea. Take a life table — the schedule of survival by age — and split each year of life into time spent in good health and time spent in poor health, using the proportion of people at each age who report themselves healthy. Add up the healthy portions and you have healthy life expectancy: the years a person can expect to live in good health under current conditions. The arithmetic, known as the Sullivan method, is simple; everything difficult is in the word "healthy".
The definitions. The Office for National Statistics publishes healthy life expectancy based on people's own rating of their general health as good or better, and separately disability-free life expectancy based on whether a long-standing condition limits their activities. Eurostat's healthy life years use a single question about activity limitation asked across the European Union. The Global Burden of Disease's health-adjusted life expectancy weights each condition by its severity from a disability schedule rather than asking anyone how they feel. Japan's official measure asks about limitation in daily life. These are four different questions, answered by different surveys, and their results are not interchangeable. A country can have a high figure on one and a middling figure on another.
The problem with asking. Self-reported health depends on culture, expectation and the language of the question. Populations with better access to diagnosis report more conditions. Older people rate their health relative to their peers. Translation changes meaning. The measures are consistent within a country over time, which is what they are mostly used for; across countries they carry a warning.
Compression or expansion. The question that matters is whether the healthy portion is growing as fast as the whole. If people live longer but the extra years are spent in poor health, morbidity is expanding; if illness is being squeezed into a shorter period at the end, it is compressing. In the United Kingdom, healthy life expectancy stopped rising around 2011 and has fallen for women since, so the years in poor health have grown; other countries show compression on some measures and expansion on others, and the answer depends partly on which measure is chosen.
Inequality. Healthy life expectancy varies far more than life expectancy does across a country. In England the gap between the most and least deprived areas is of the order of two decades of healthy life against about a decade of life, which is why healthy life expectancy is the measure that appears in policy targets and why its stalling is a policy story as well as a demographic one.
Reading a figure. Which definition, which survey, which question; whether the comparison is across time within one series, which is usually sound, or across countries or definitions, which usually is not; and whether the change is in the health portion or in the life table beneath it, because a pandemic year lowers healthy life expectancy by lowering survival, not by changing anyone's health.