Possible CM set configuration
    

Case

Controls

pr(case)

Case RRT status

pr(controlscase)

Weight^{a}


9

18, 24

λr(Z
_{9};β)

+

1/12 × 1/7

8/2

18

9, 24

λr(Z
_{18};β)



2/(8 × 7)

12

24

9, 18

λr(Z
_{24};β)

+

1/7 × 1/12

8/2

 The calculation requires the probability of each of the possible case/control combinations, given that subjects 9,18, and 24 are in the countermatched 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(Z
_{9};β) is the rate ratio based on the covariate value, and is the rate ratio parameter. pr(controlscase) are control selection β probabilities. For example, for subject 18, because she is RRT, pr(controls = 9,24case = 18) is the probability of sampling two RRT+ from the 8 in the risk set. ^{a}Weights for analysis equal to pr(controlscase) × 12 × (8 × 7/2).