Identifying possible sources of bias introduced in traffic safety research : comparison of blind linkage with volunteer clinical samples.

In research on alcohol, drugs and traffic safety, and on road safety in general, investigators have employed a variety of measures to construct samples for research. Differing sample construction procedures may introduce bias into the resulting samples, but this possibility has rarely if ever been assessed empirically. In this research we compare two samples of individuals who obtained treatment for a substance abuse problem. One sample was obtained by blind linkage procedures, that is, groups were identified in the clinical records. A second sample was identified in the clinical records, and then tracked, contacted, and asked to consent to participate in a research study. Comparisons of the two samples on psychosocial and problem indicators derived from clinical records revealed a large number of significant differences between the samples. In all cases, the differences indicated that the group tracked and asked for consent had higher levels of functioning than the blind linkage group. The possible reasons for these differences, and their implications for research and research policy, are discussed. (Author/publisher) For the covering abstract of the conference see ITRD Abstract No. E201067.