Farr exploited his GRO post compiling abstracts in a way that went beyond the original job description. In so doing he applied the techniques of Benjamin Gompertz (the Gompertz curve), and the closely allied statistical "law of mortality" of Thomas Rowe Edmonds. Farr, by relying on the existing mathematical model of mortality, could use data sampling to cut back the required computation. From the GRO data he constructed a series of national life tables.
The theory of zymotic disease was Farr's contribution to the debate on aetiology. He identified urbanisation and population density as public health issues. In terms of nosology he classed epidemic, endemic and contagious diseases as "zymotic", seen as diseases of filth and overcrowding.