Contemporaneous spatial sampling at scenes of crime: Advantages and disadvantages

Sample number and spatial distribution are critical in obtaining the best comparison of suspect to scene material, be it soil, rock fragments/dust, vegetation or other transferable anthropogenic materials (paint, glass, tar). This is illustrated using a case study, in which a scene of crime was examined and sampled for possible transferable materials. A sample was collected by a Scenes of Crime Officer (or CSI: Crime Scene Investigator); the authors collected 112 grided samples. Replicated suspect items comprised the author's shoes, which were seized and comparison made to the scene samples. The single, CSI-collected sample allowed only minimal comparison to the 'suspect' samples. The grid of 112 samples showed a wide variation in comparison, with samples from one particular 16 m2 area, or roughly 1/3 of the total sample area showing better comparison to footwear samples than the remaining 2/3 of the sample grid. This was achieved by counting the number of fragments of plasterboard in samples, but could be achieved by any other numerical measurement method. Spatial interpolation was used to convert the information collected from point locations at the suspect scene to a continuous spatial map so that any spatial patterns could be observed. The implications are that where inhomogeneity is observed or suspected at a scene, then sufficient samples to map this variation must be taken. Once sampled however, distributions of materials may have the potential to place a suspect to within a few meters of a scene.