  In observational studies for the interaction between exposures on dichotomous
outcome of a population, one usually uses one parameter of a regression model
to describe the interaction, leading to one measure of the interaction. In this
article, we use the conditional risk of outcome given exposures and covariates
to describe the interaction and obtain five different measures for the
interaction in observational studies, i.e. difference between the marginal risk
differences, ratio of the marginal risk ratios, ratio of the marginal odds
ratios, ratio of the conditional risk ratios, and ratio of the conditional odds
ratios. By using only one regression model for the conditional risk of outcome
given exposures and covariates, we obtain the maximum-likelihood estimates of
all these measures. By generating approximate distributions of the
maximum-likelihood estimates of these measures, we obtain interval estimates of
these measures. The method is presented by studying the interaction between a
therapy and the environment on eradication of Helicobacter pylori among
Vietnamese children.
