on the absolute risk scale. Even if D is not rare, if one is looking at hazards across instantaneous time the event is rare in each small time interval. Thus if λAB(t) is the instantaneous hazard rate at time t for those exposed to both A and B, then non-interaction would correspond to: (λAB(t)−λAB¯(t))=(λAB¯(t)−λAB¯(t))+(λA¯B(t)−λAB¯(t)), that is, additivity of hazards. This additivity then corresponds to a multiplicative model in cumulative survival. The notion here, biologically, might be that these two exposures act through completely disjoint biologic pathways.
