Estimating the size of the drug dealing population to describe the drug market: A comparison between different metropolitan areas in Italy

Speaker: Carla Rossi (University of Rome Tor Vergata, Center for Biostatistics and Bioinformatics, Itálie).

We estimate the size of the population of street dealers constituting the labour force of the retail drug market in different metropolitan areas in Italy, by applying capture-recapture methods. The estimate is based on the Archive of the Central Direction of Criminal Police of the Ministry of the Interior and, specifically, on the database of subjects registered for breaching the drug law (art. 73). This is the first time that this population has been estimated using statistical models anywhere in the world. In particular, this is the first application of capture-recapture techniques to this particular population; a first step to evaluate the impact of enforcement drug policies. It therefore provides a very useful tool for planning more effective interventions. Most problem drug users become involved in dealing drugs, therefore it is important to try to prevent this step, or at least to help give these drug users an alternative to jail, with the aim of reintegrating them in the society; A single source capture -recapture model was applied. Chao and Zelterman’s estimators were obtained. A bootstrap simulation to calculate the variance of the estimators was carried out, and the confidence interval was obtained. Males and females, divided by age and class, have been treated separately, and the years between 2005 and 2008 considered, in order to allow comparisons with the estimates of users obtained on the basis of the database of subjects registered for breaching the drugs law (art. 75). In some sense we can call these latter “street drug users”. Studying the population through these years results is particularly interesting, because of the change of law in 2006 which, from a statistical point of view, produced a modification in the “case definition.” Before 2006, possession of illicit drugs was prosecuted in accordance with the principle of mandatory prosecution, independently of the quantity of the substance. The hypothesis is that the new case-definition produces a displacement of subjects from the consumers archive (art. 75) to the dealers archive (art. 73), and that this phenomenon is wider in the main metropolitan areas. This is why we studied the biggest drug-user population (cannabis users) using a different database of the Ministry of the Interiors and presented the results in a different contribution. The data-set has been provided by the Italian Ministry of the Interiors and comprises several variables for each subjects. The different subjects are univocally identified, so that it is possible to count the number of captures in a specified period. The areas considered in this contribution are: Rome, Milan, Turin, Genoa, Bologna, Naples, Bari, Palermo and Cagliari which comprise (in 2005) about 36% of the population in the age group 15-64, but, according to the estimates obtained for these areas for the same period, cannabis users from these areas comprise about 44% of the total estimated for Italy. In the conclusion we will focus on some challenges in interpretation of the results. We will also focus on the challenges and the opportunities of using different archives and surveys. A by-product of the analyses is the description of the main trends in the retail market and the analysis of the correlation among these and the main trends in use. One focus will be poly-dealing, which is the other side of the coin of poly-use. This might also be due to the new law implemented in 2006, which will consider all illegal substances at the same level.

CV: Carla Rossi´s current position: Professor of Medical Statistics, University of Rome Tor Vergata; project manager of the research project, financed by the Open Society Institute: Illicit Drug Market and its Possible Regulation; member of the management board of the European Monitoring Center for Drugs and Drug Addiction (EMCDDA) as representative of the European Parliament; Career: Professor Rossi has had a long career in the field of statistics and illicit drug research, and has been involved in several groundbreaking projects that examine illicit drug markets, incidence and prevalence of drug use. Since 1981 she has held several academic positions, including Professor of Statistics at the University of Rome. She is the author of more than 150 scientific publications and monographs, and 40 educational papers and books on subjects including mathematical models for life sciences, biometry and biostatistics, mathematical models and statistical analysis for decision problems in social and health care policy and forensic sciences.