© 1996 Oxford University Press
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The Lack of Selection Bias in a Snowball Sampled Case-Control Study on Drug Abuse
*Instituto de Medicina Social, State University of Rio de Janeiro R. Sao Francisco Xavier 524, 7 andar, 20559-900 Rio de Janeiro-RJ, Brazil
**London School of Hygiene and Tropical Medicine, University of London UK
Lopes C S (Instituto de Medicina Social, State University of Rio de Janeiro, R. Sao Francisco Xavier 524, 7 andar, 20559-900 Rio de Janeiro-RJ, Brazil), Rodrigues L C and Sichieri R. The lack of selection bias in a snowball sampled case-control study on drug abuse. International Journal of Epidemiology 1996; 25: 1267–1270.
BACKGROUND: Friend controls in matched case-control studies can be a potential source of selection bias based on the assumption that used the snowball sampling method based on friendship for the selection of cases and controls.
METHODS: The cases selected for the study were drug abusers located in the community. Exposure was defined by the presence of at least one psychiatric diagnosis. Psychiatric and drug abuse/dependence diagnoses were made according to the Diagnostic and Statistical Manual of Mental Disorders (DSM-III-R) criteria. Cases and controls were matched on sex, age and friendship. The measurement of selection bias was made through the comparison of the proportion of exposed controls selected by exposed cases (p1) with the proportion of exposed controls selected by unexposed cases (p2). If p1 = p2, then, selection bias should not occur.
RESULTS: The observed distribution of the 185 matched pairs having at least one psychiatric disorder showed a p1 value of 0.52 and a p2 value of 0.51, indicating no selection bias in this study.
CONCLUSION: Our findings support the idea that the use of friend controls can produce a valid basis for a case-control study.
Keywords Case-control studies, snowball sampling, selection bias, friendship matching, drug abuse
Revised 1 May 1996
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