This animation depicts how odds ratios are calculated, first by comparing two populations, one of which has had an exposure to radiation and the other of which has not been exposed, then following both populations to see who gets cancer, then calculating an odds ratio based on the number of cancer cases in these two populations.

The animation opens with two populations of ten people each. Population A is exposed to I-131, a carcinogen. Population B has no exposure. Three people in Population A develop cancer, while only one person in Population B develops cancer.

The ensuing odds ratio table breaks out the numbers: in Poplation A, on the top row of the table, three people have cancer and seven have no cancer; in Population B, on the bottom row of the table, one person has cancer and nine have no cancer. In calculating the odds ratio, three is multiplied by nine and seven is multiplied by one. Dividing out the resulting fraction, twenty-seven over seven, results in 3.86, meaning that people in Population A are almost four times as likely to develop cancer as people in Population B.