  In epidemiological research, it is common to investigate the interaction
between risk factors for an outcome such as a disease and hence to estimate the
risk associated with being exposed for either or both of two risk factors under
investigation. Interactions can be estimated both on the additive and
multiplicative scale using the same regression model. We here present a review
for calculating interaction and estimating the risk and confidence interval of
two exposures using a single regression model and the relationship between
measures, particularly the standard error for the combined exposure risk group.
