We illustrate the decomposition of variance using an IRT measurement model with data from the Netherlands Twin Registry (NTR; Boomsma et al. 2002b). Attention problems were measured with the Young Adult Self-Report (YASR; Achenbach 1997). We used data collected in the year 2000 from 460 males and 966 females from MZ twin pairs, 288 males from DZ same-sex twin pairs, 561 females from DZ same-sex twin pairs, and 305 males and 441 females from opposite sex twin pairs. All twins were between 18 and 30 years (inclusive). All available data were used, including data from incomplete pairs and individuals with several items missing. It was assumed that data were missing at random (cf. Van den Berg et al. 2006c).
