Fig. 2.

ICCs and 95% CIs in relation to math and reading achievement tests for SS-M, SS-F, and OS samples of younger (grades 3–5) and older (grades 6–8) twins and closely spaced siblings. Closely spaced sibling pair is defined as two siblings having the same mother for whom the distance in months between births is the smallest among births to this mother between 1994 and 2002. ICCs are based on multilevel mixed-effects linear regression estimated with maximum likelihood, where within-individual across-grades errors are assumed to have an autoregressive structure of order one. Random effects are structured at the twin/sibling pair and individual levels. ICC is computed as the ratio of between-pair variation to the sum of within- and between-pair variations. n = 299,426 children (24,640 twins and 274,786 singletons) and 1,796,532 children-year observations. F, female; M, male; Read, reading; Sib, sibling.