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Quantifying experience in sample size determination for drug analysis
Linköping University, Faculty of Arts and Sciences. Linköping University, Department of Mathematics, Statistics .
2006 (English)In: Law, Probability and Risk, ISSN 1470-8396, Vol. 4, no 4, 217-225 p.Article in journal (Refereed) Published
Abstract [en]

 Forensic analysis of pills suspected to contain illegal drugs is a time-consuming process; therefore, only a small sample from a seizure can be investigated. Notwithstanding, for drugs like Ecstacy, experience of forensic analysts indicates that a seizure of tablets usually consists either wholly of illicit drugs or no illegal substances at all. Consequently, it should be possible to draw fairly accurate conclusions based on very small samples, if all pills in a sample are indeed analytically identical. The forensic experience is modelled using a beta prior distribution for the proportion of drug-containing tablets in a seizure, and the sample size is determined so that a certain confidence statement can be made about this proportion. The parameters of the beta prior must be set to correspond with the experience, and this paper suggests a method for estimating these parameter values from a database comprising analyst reports representing the experience. The technique is applied to proportions of Ecstacy pills, and the results show that a sample of five pills is enough to state with a high level of confidence that at least half the tablets in a presumed Ecstacy seizure are genuine.

Place, publisher, year, edition, pages
2006. Vol. 4, no 4, 217-225 p.
Keyword [en]
drug sampling; prior distribution; likelihood; resampling
National Category
URN: urn:nbn:se:liu:diva-37175Local ID: 33862OAI: diva2:258024
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2011-01-11

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