The PEER variable used here was developed from the Important People and Activities (IPA) Scale, an instrument with retest reliabilities, as well as measures of validity that approach .80 (Longabaugh et al., 1993, 2001). While prior analyses used PEER drinking quantity because LR is more closely linked to quantities consumed per occasion, ALSPAC currently only records PEER substance frequencies. Therefore, a latent variable was created reflecting the subjects’ perception of whether the relevant peer used alcohol, nicotine, cannabinols, and/or other illicit drugs. In the IPA, four individuals felt by the subject to be important peers were noted, with subsequent questions regarding the perceived use of substances yielding a mean number of peers using each of the four substances.
