The informational odds ratio (IOR) measures the post-exposure odds divided by the pre-exposure odds (ie, information gained after knowing exposure status). A desirable property of an adjusted ratio estimate is collapsibility (ie, the combined crude ratio will not change after adjusting for a variable that is not a confounder). Adjusted traditional odds ratios (TORs) are not collapsible. In contrast, Mantel-Haenszel adjusted IORs generally are collapsible. IORs are a useful measure of disease association in environmental case-referent studies, especially when the disease is common in the exposed and/or unexposed groups.