The calculation requires the probability of each of the possible case/control combinations, given that subjects 9,18, and 24 are in the counter-matched set. pr(case) is the probability that the subject is a case based on the proportional hazards model. For instance, for subject 9, λ is the baseline hazard, r(Z9;β) is the rate ratio based on the covariate value, and is the rate ratio parameter. pr(controls|case) are control selection β probabilities. For example, for subject 18, because she is RRT-, pr(controls = 9,24|case = 18) is the probability of sampling two RRT+ from the 8 in the risk set. aWeights for analysis equal to pr(controls|case) × 12 × (8 × 7/2).