Data from Cole et al. [4] are used in the follow- ing example (Table 1). Oligoclonal band status, an immunological indicator treated as the exposure variable, was determined to be normal in 38 and abnormal in 38 patients who were followed for a maximum of 5 years or until onset of multiple sclerosis, the outcome. The odds ratio and risk ratio are measures of association for 2×2 tables, defined as: