
 The
last step is to calculate the risk ratio using the information from steps 1
and 2.
 Since now that the exposure data involve persontime and each individual
is considered as a single stratum, the MantelHaenszel 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).
