2.5 The importance of data definitions

Gapminder defines life expectancy at birth as the average number of years a newborn child would live if current mortality patterns were to stay the same. In comparing values for different countries over time, the assumption is made that the estimates for each country for each year have been prepared in the same way, that definitions have been applied consistently, and that data collection and reporting have been standardised. Older data are likely to be less reliable and more uncertain. Infant mortality rates have dropped substantially, even in more recent times, having a strong positive effect on life expectancy at birth.

Comparing countries over time also assumes that the countries have remained the same. This is definitely not true, as Gapminder points out. They make the default assumption that borders have always been as they are now. (Their webpage includes an instructive video of how borders have changed since 1800.) Examples of large changes include Germany, the countries in the former Soviet Union, and the Indian subcontinent.