Initial analyses were conducted to examine whether friends' alcohol use was associated with an increase in adolescent quantity of alcohol use and problem alcohol use. As each twin is part of a twin pair, when conducting epidemiological analyses reduced variance due to the correlation between twins' scores can cause high false positive rates [33]. To account for this the data were treated as equivalent to a two-stage cluster design, with twin pairs as the primary sampling unit [34], using the survey analyses procedure in STATA (release 9) [35].
