 Steel’s own reconstruction of how extrapolation works in the biomedical sciences is called comparative process tracing. He assumes that causes C (such as medical interventions or the ingestion of toxic substances) bring about their effects E (such as the appearance of symptoms or improvements or deteriorations of symptoms) through a series of steps or stages. To trace a causal process means to investigate through what set of stages C brings about E. Process tracing is comparative when the set of stages through which C brings about E in one species or population is compared to the set through which it does so (if it does so indeed) in another.