- The last step is to calculate the risk ratio using the information from steps 1 and 2. 

- Since now that the exposure data involve person-time and each individual is considered as a single stratum, the Mantel-Haenszel method for rate ratio will be used to calculate the risk ratio (Rothman and Greenland, Modern Epidemiology, 1998, page 270). Remember that , for individual i, his/her observed odds is (ai, bi) which is either (1:0) or (0:1), and the expected odds is (xi, yi). Algebraically, the numerator of the risk ratio is the summation of yi over the individuals whose observed odds were (1:0), and the denominator of the risk ratio is the summation of xi over the individuals whose observed odds were (0:1).