Figure. Examples of Methods for Defining ASD-Discordant and ASD-Concordant Sibling Pairs.

ASD indicates autism spectrum disorder. Each sibling is followed from birth to end of follow-up (death, emigration, or end of follow-up) or ASD diagnosis. Analysis methods A and B agree for the clear majority of all sibling pairs. If S2 was not observed with ASD, the concordance or discordance status would be the same for both methods. In the Figure, for the family with 2 siblings where S1 is diagnosed with ASD in 1993 and S2 in 1998, the sibling pair (S1, S2) will be discordant in 1993 because S2 is censored at the ASD diagnosis of S1. However, the pair (S2, S1) will be concordant in 1998. The 2 pairs are 2 candidates representing the family. For calculating heritability, 1 of these representative sibling pairs was randomly selected. As a consequence, the algorithm led to a loss of about half of the concordant pairs compared with results under the assumptions and methods applied in the alternate method (analysis method B), in which calculating heritability typically does not consider sibling pairs as both discordant or concordant depending on which sibling is considered dependent, but instead follows them as a pair.