Demography

Demography is the quantitative study of human populations, their size, age and sex composition, and the birth, death and migration rates that drive how those populations change over time. The discipline's founding work is John Graunt's Natural and Political Observations Made upon the Bills of Mortality, published in London in 1662, in which a haberdasher with no formal scientific training analyzed the city's weekly death registers and found regular, predictable patterns in causes of death and seasonal mortality, showing for the first time that population level regularities could be extracted from what had looked like unconnected individual records.

Religious demography, how many adherents a tradition actually has, where they live, and whether that number is growing, shrinking or migrating, is a claim nearly every religious group makes about itself, and it is frequently inflated, outdated or based on self-report rather than independent count. The atlas treats population figures drawn from national census data, government religious registration and peer reviewed demographic surveys as a different and generally more reliable category of evidence than a tradition's own membership claims, and flags the difference where the two figures diverge.