 Steel breaks the extrapolator’s circle by developing a mechanisms-based extrapolation strategy—the strategy of comparative process tracing. Once a mechanism for some phenomenon has been elucidated in a model (such as a particular process of carcinogenesis in rats), scientists (toxicologists in this case) then compare key stages (particularly downstream stages) of the model with the stages in the target, paying particular attention to points in the process where differences are most likely to arise. The greater the similarities of the entities, activities, and organization of the mechanisms in both populations, the stronger is the basis for extrapolation; the greater the differences, the weaker the basis (but see Howick et al. 2013; see also the sections on extrapolation in the entries on  molecular biology and  experiment in biology).