37. Formulation of a hypothesis Method of difference; if the disease frequency is significantly different between two sets of circumstances the disease might have a causal association with a particular factor that differs between the two
38. Method of agreement; If single factor is common in a number of circumstances in which the disease occurs causal association can be suspected.
39. Method of concomitant variation If a factor varies in proportion to the frequency of disease causal association can also be suspected.