CONTRADICTORY SUBJECT RESPONSE IN LONGITUDINAL RESEARCH

Objective: This study investigated the prevalence of logical and estim ation errors in a 4-year longitudinal survey of substance abuse for el ementary and junior high school students. Logical errors occur when a subject reports having used a substance on one measurement occasion, b ut reports never having used the substance on a second measurement occ asion. Estimation errors occur when a subject reports a lifetime frequ ency of use on a later survey that is less than the lifetime frequency of use reported for an earlier survey. Method: Substance abuse survey s were completed every year for four years by predominantly black Dist rict of Columbia students, beginning when the students were in 4th/5th grade, and ending when they were in 7th/8th grade (Ns ranged from 5,6 71 to 6,421). Errors for alcohol, cigarette and marijuana use were cal culated for every pair of years. Background characteristics such as de mographic and academic performance variables were also assessed for ea ch of the four years. Results: Logical errors decreased for all substa nces as students got older, whereas estimation errors remained stable. Marijuana had the highest frequency of logical errors in every pair o f years except one, and had the lowest frequency of estimation errors. Students who made errors in Years 1-2 were 1.64 times more likely to make errors again in Years 3-4. For some of the years, students who ma de any kind of error had lower school grades, were judged untrustworth y by their peers and were more Likely to be males. Conclusions: The al arming prevalence of errors should be considered when conducting longi tudinal substance abuse research. Several methods of dealing with thes e errors are reviewed. In most cases, the best a researcher can do is assess the error rates and report them.