Table 1. Distribution of the sample of sib pairs in cross table according to exposure of sib 1 and disease status of sib 2.
| Sib 1 (affected) | |||
|---|---|---|---|
| Sib 2 | Exposed | Unexposed | |
| Affected | a | b | a+b |
| Unaffected | c | d | c+d |
| a+c | b+d | N | |
| KSE=a/(a+c) | KSĒ=b/(b+d) | KS=(a+b)/N | |
The sibling recurrence risk over the whole sample (KS) and sibling recurrence risks stratified on sib 1's exposure (KSE and KSĒ) can be derived from the observed numbers (a, b, c and d). The odds ratio of recurrence (ORR) is equal to:
 |