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Composite sampling can be more cost effective than simple random sampling. This paper considers how to determine the optimum number of increments to use in composite sampling. Composite sampling terminology and theory are outlined and a model is developed which accounts for different sources of variation in compositing and data analysis. This model is used to define and understand the process of determining the optimum number of increments that should be used in forming a composite. The blending variance is shown to have a smaller range of possible values than previously reported when estimating the number of increments in a composite sample. Accounting for differing levels of the blending variance significantly affects the estimated number of increments.

Generally, biological samples are collected from crime scenes using swabbing, cutting, or taping techniques. However, these methods are limited in their abilities to recover diluted, masked, or otherwise invisible stains. Additionally, their targeted nature allows only a small portion of a larger stain to be collected at one time. In this study, a sterile wet-vacuum collection system was evaluated in its ability to collect small volume bloodstains from various substrates. Vacuuming was compared to swabbing and taping methods currently used in forensic analysis. Samples were collected from porous and nonporous surfaces; the efficacy of each collection method was evaluated with a colorimetric presumptive blood test. To evaluate each collection method, dilutions containing from 0.25 nl to 25 μl human blood were spotted on common substrate materials, allowed to dry, and recovered. For comparison to the novel method, single-swabbing and tape-lifting techniques were performed in this study to collect samples for presumptive testing. During wet vacuum collection, stains were saturated with sterile buffer and suction was applied to the surrounding area, accumulating buffer in a collection bottle. Collected buffer was then filtered through membranes to capture cellular material, which were then presumptively tested for the presence of blood. Testing was performed with Kastle-Meyer (phenolphthalein) reagents. Each sample was photographed under consistent conditions in order to determine signal intensity. It was shown that the wet-vacuuming technique is able to recover sufficient amounts of blood for presumptive testing from multiple substrates. This method was able to detect similar dilutions of blood as traditional techniques in samples collected from porous surfaces, but was less effective on a nonporous substrate. Presumptive test image analysis shows increased relative intensity in collections from textiles, such as denim, when using the wet-vacuum system. Considering the results of a contemporaneous DNA quantification study, it was shown that in instances where a very weak presumptive result is found, the wet-vacuum technique may be better able to collect genetic material for downstream processing than the traditional methods evaluated. This study demonstrates the potential of wet-vacuuming as a suitable alternative technique to collect adhered cellular material from substrates in forensic investigations.

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