The characterization of inconsistencies in self-reports of alcohol and marijuana use in a longitudinal study of adolescents.

The reliability of self-reported measures remains an important issue for research on adolescent alcohol and drug use. Many studies have concluded that adolescents' self-reports are valid and reliable, but few studies have excluded consistent nonusers from their reliability estimates, and no study has examined in detail the reliability of reported age at first use of substances. This study explores the consistency of self-reports of frequency of use and age of first use of alcohol and marijuana in a sample of 5,770 secondary school students in a southeastern U.S. county. Two waves of data were collected between 1985 and 1988 using state-of-the-art data collection procedures and self-administered instruments. Consistency of reports was examined by comparing reports at T1 and T2, approximately 1 year apart. Results showed that when consistent nonusers were dropped from the analysis, consistency rates of lifetime frequency of use dropped from 82.7% to 74.7% for alcohol and from 95.6% to 83.2% for marijuana. Reports were more consistent for lifetime marijuana use than for alcohol use, but these results must be interpreted with caution given differences in the measures for the two substances. Reliability of reported age of first use was very low for both substances. When consistent nonusers were dropped from the analysis, only 27.8% of respondents made consistent estimates of their age at first alcohol use and 34.4% for their age at first marijuana use. Implications and recommendations for this area of research are discussed.