https://www.sciencedaily.com/releases/2019/10/191008140406.htm

Declining fertility rates and increased life expectancy has resulted in increasing numbers and proportions of older people in populations across the world. Policymakers, as well as public and private healthcare providers need to understand how healthy the older population is to, for example, plan for future healthcare facilities and answer questions related to the employability of older individuals, which is in turn relevant in the context of adapting pension systems. Since most information for these topics comes from self-reported health data, it is important to understand how reliable this data is, especially in the context of population aging in Europe where governments are increasingly concerned with the health of the population, the care of the elderly population, and the comparison of health measures between countries.

Self-reported data is collected when individuals are simply asked to tell an interviewer about how they see their own health status. This method requires less time and is more cost effective than tested data, which is why it is much more readily available. Tested health data on the other hand, requires a nurse, other health professional, or trained interviewer to collect data through, for instance, cognitive and physical performance tests or blood samples that require more time and effort.