Combining participants from both interviews yielded a total sample size of 6,383 twins, of which 4,520 twins (71%) completed both the 1996-2000 and the 2004-2007 interview. The majority (73%) of twins who completed the 1996-2000 interview also completed the 2004-2007 interview, and almost all (95%) of the twins who completed the 2004-2007 interview also completed the 1996-2000 interview. The sample consisted of 1,502 monozygotic (MZ) female twins (MZF), 1,135 MZ male twins (MZM), 1,183 dizygotic (DZ) female twins (DZF), 991 DZ male (DZM) twins, and 1,572 DZ opposite-sex twins.
