an example of how to calculate the risk ratio when the control data were
from the past. For each subject the observed odds and expected odds are
listed. The numerator of the risk ratio is the sum of y’s over the subjects
whose observed odds were (1:0), that is the first subject and the fourth
subject (20+19=39). The denominator of the risk ratio is the sum of x’s over
the subjects whose observed odds were (0:1), that is the second, the third
and the fifth subject in this example. So the denominator is 8+5+12=25. Thus
the risk ratio is 39/25 = 1.56.