A measure of the odds of an event happening in one group compared to the odds of the same event happening in another group. In cancer research, relative odds are most often used in case-control (backward looking) studies to find out if being exposed to a certain substance or other factor increases the risk of cancer. For example, researchers may study a group of individuals with cancer (cases) and another group without cancer (controls) to see how many people in each group were exposed to a certain substance or factor. They calculate the odds of exposure in both groups and then compare the odds. A relative odds of one means that both groups had the same odds of exposure and, therefore, the exposure probably does not increase the risk of cancer. A relative odds of greater than one means that the exposure may increase the risk of cancer, and a relative odds of less than one means that the exposure may reduce the risk of cancer. Also called odds ratio.