The value of a scientific analysis and the significance of the results are influenced by the degree to which the sample analyzed reflects the composition of the material or population under study. An appropriate methodology will result in representative sampling. Formulae for representative sampling protocols were evaluated, and an optimal formula to be used in a sampling plan for “street” drug exhibits with a large number of units was selected. The selected formula was applied to choose a number of sample units for routine chemical analysis of controlled substances, with the analytical result considered to be representative of the entire lot of powdered drug units in the exhibit. The results obtained during the research demonstrate that adoption of the formula n = 20 + 10%(N − 20) [where n = sample size, N = total population (N ⩾ 20)] can be used for selection of an appropriate representative sample from powdered street-drug exhibits. This sampling procedure results in forensic analyses of controlled substances which are supportable to a reasonable scientific certainty, and which are obtained in an efficient manner.