Estimating the Prevalence of Illicit Drug Use Among Students Using the Crosswise Model

Abstract

Objective: The aim of our study is to compare the prevalence of illicit drug use estimated through a technique referred to as the “crosswise model” (CM) with the results from conventional direct questioning (DQ). Method: About 1,500 students from Tehran University of Medical Sciences 2009–2010 were first interviewed by DQ and, then three months later, by the CM. Result: The CM yielded significantly higher estimates than DQ for lifetime prevalence of use of any illicit drug (CM = 20.2%,DQ = 3.0%, p < .001) and for lifetime prevalence of use of opium or its residue (CM = 13.6%, DQ = 1.0%, p < .001). Also, for use of any illicit drug in the last month and use of opium or its residue in the last month, the CM yielded higher point estimates than DQ, although these differences were not significant (any drug: CM = 1.5%, DQ = 0.2%, p = .66; opium: CM = 3.8%, DQ = 0.0%, p = .21). Conclusion: Our findings suggest that the CM is a fruitful data collection method for sensitive topics such as substance abuse.

Keywords :

• sensitive questions
• illicit drug use
• substance abuse
• crosswise model
• randomized response technique
• survey methodology
• Iran

THE AUTHORS

Mansour Shamsipour received the master's degree in epidemiology from the School of Public Health, Tehran University of Medical Sciences at Tehran, Iran, where he is currently a PhD student in epidemiology. He worked for two years in applying nonrandomized response (NRR) models for analyzing sensitive questions. His research area includes designing of Randomized Controlled Trial (RCT), sensitive research designs and environmental & occupational epidemiology. His current interest is exposure assessment modeling in air pollution to understand the public health impact of air pollution.

Masud Yunesian, MD, PhD, is Professor of Epidemiology at the Institute for Environmental Research and Department of Environmental Health, School of Public Health, Tehran University of Medical Sciences. He received the MD and PhD degrees in epidemiology from Tehran University of Medical Sciences. His main interest is methodology of epidemiological studies, especially in the field of health effects of air pollution. He has also been involved in a couple of studies on epidemiology of mental health problems.

Akbar Fotouhi, MD, PhD, is Professor of Epidemiology at the School of Public Health, Tehran University of Medical Sciences. He received the MD and PhD degrees in epidemiology from Tehran University of Medical Sciences. His main interest is methodology of epidemiological studies. He has been involved in some researches on assessing sensitive issues with focus on their methodological aspects. His current main research area is ophthalmic epidemiology and has run some large population-based studies in this area. His two main studies are Tehran Eye Study and Shahroud Eye Cohort Study with many published reports.

Ben Jann, Dr., is an Associate Professor of Sociology at the University of Bern, Switzerland. He received the doctoral degree in sociology from ETH Zurich in Switzerland. His research interests include social-science methodology, statistics, social stratification, and labor market sociology. Recent publications include papers in Sociological Methodology, Sociological Methods & Research, Public Opinion Quarterly, the Stata Journal, and the American Sociological Review.

Afarin Rahimi-Movaghar, MD, MPH, is Associate Professor of Psychiatry and Head of the Iranian National Center for Addiction Studies at Tehran University of Medical Sciences. Her main interests are in the fields of health service research and epidemiology. She has directed several national surveys and has served in several national-level positions including as the Director of the Offices on Mental Health Services in the Iranian Ministry of Health, as well as the National Mental Health Research Department. She has been a Consultant for several programs in the Middle East region countries and a Member of several advisory committees for the World Health Organization.

Fariba Asghari MD is an Associate Professor at the Medical Ethics and History of Medicine Research Center of Tehran University of Medical sciences. Her research is mainly focused on medical error, professionalism, and ethics education. She is currently doing two other projects using indirect approaches for estimating prevalence of illegal abortion and publication misconduct.

Ali Asghar Akhlaghi, is a PhD student in Biostatistics from Tehran University of Medical Sciences, His research area includes cure models in survival analysis, cross over & sequential trial analysis, and generalized additive models (GAMs). He received the master's degree in biostatistics from the School of Public Health, Tehran University of Medical Sciences at Tehran, Iran.

Notes

1 The wording of the questions was (translated from Persian): “Did you understand the instructions to fill out the questionnaire?” with answer categories “Completely”, “To some extent”, and “Did not understand” and “To what extent do you believe that this questioning technique protects your anonymity?” with answer categories “Highly”, “Moderately”, “Little” and “Not at all”.

2 Note that the Crosswise Model is formally identical to Warner's original RRT scheme Warner, S.L., 1965. Randomized response: A survey technique for eliminating evasive answer bias, Journal of the American Statistical Association 60: 63–69.

3 Note that in the DQ study, the estimates for lifetime and last month use of any illicit drug are based on an index from separate questions for different types of drugs (the same types as listed in the question used in the CM study).

4 This is a number known to students in Iran because the number is used in many contexts, for example as the password for enrolling in classes.