NCJ Number
139763
Journal
Journal of Forensic Sciences Volume: 37 Issue: 6 Dated: (November 1992) Pages: 1541-1549
Date Published
1992
Length
9 pages
Annotation
To facilitate the use of a random sample of a total drug exhibit to use as evidence of the qualitative characteristics of the total sample, this article presents a statistical basis for determining a lower limit for the proportion of units in a population which contains a drug, at a given confidence level.
Abstract
In Israel the two most common drugs of abuse are heroin and hashish. Street doses of heroin are often packaged in small pieces of folded paper, which are further wrapped in plastic that is heat sealed; it is thus time consuming to open and weigh the enclosed powder. There are mixed legal precedents as to whether or not the qualitative result from sampling applied to the total exhibit. Only one precedent at the district court level exists in Israel regarding the acceptance of the calculated weight of a drug exhibit based on sampling; this precedent was not favorable to the prosecution. New legislation has been proposed to provide that the qualitative results and calculated weight for the total exhibit based on random sampling can be evidence as applied to the total exhibit. The burden of proof to contradict this then shifts to the defendant. This article deals with the statistical basis for the new legislation. The statistical analysis presented uses binomial and hypergeometric distributions to determine a lower limit for the proportion of units in a population that contains a drug. A method for calculating the total weight of a drug present in a population within a given confidence interval is also presented. Under this analysis, in the event of no failures (all units sampled contain a drug), a sample size of six or seven units is generally sufficient to state that a proportion of at least 0.70 of the population contains a drug at a confidence level of at least 90 percent. When failures do occur in the sample, point estimation is used as the basis for selecting the appropriate sample size. 5 tables and 11 references