Here is 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.