the two exposures mutually enhance each other and under the other they are seen as working against each other. So the direction of the finding can be different, depending on the selected model for nonindependence. Another consideration is that power for detecting “interaction” can be markedly greater if the null is specified additively. This enhancement of statistical power happens, not for some subtle reason, but simply because super-multiplicative joint action of two risk-enhancing factors is more distant from an additive null than from a multiplicative null.
