The subject of early (for our purposes, pre-World War II) data on mortality is a vast one, and thus this treatment is quite broad.

The emphasis is on identi­fying classes of data, sources of ambiguity, and gen­eral approaches to problems of interpretation. Wher­ever possible, citations are made to critical surveys of the literature, rather than to the literature itself. Some of the points discussed here can be extended, with appropriate caution and revision, to the equally important, but much less tractable area of early morbidity data.