Imagine there is a rare disease, afflicting, say, only one in many thousands of adults in a country. Imagine we suspect that being exposed to something (say, having had a particular sort of injury in childhood) makes one more likely to develop that disease in adulthood. The most informative thing to compute would be the risk ratio, RR. To do this in the ideal case, for all the adults in the population we would need to know whether they (a) had the exposure to the injury as children and (b) whether they developed the disease as adults. From this we would extract the following information: the total number of people exposed to the childhood injury, formula_1 out of which formula_2 developed the disease and formula_3 stayed healthy; and the total number of people not exposed, formula_4 out of which formula_5 developed the disease and formula_6 stayed healthy. Since formula_7 and similarly for the formula_8 numbers, we only have four independent numbers, which we can organize in a table: