Comparison is the central method in forensic science and is predicated on assumptions about the items or phenomena to be compared: The comparisons need to be structurally consistent, within shared systems of relationships, and have traits that are diagnostic. Typically, one or more sources (of known origin or relationship) are compared with one or more targets (of an unknown source) by mapping identified (and, therefore, classified) traits between them. The stronger the map, both in number and complexity of traits and relationships, the stronger is the conclusion that can be drawn about the shared relationship between the source and the target. Most things in the world are different when compared and, therefore, more salient differences are found between otherwise similar items.
