Class vs. Individual Characteristics
Criminalists from many different forensic disciplines are routinely asked to determine if an item of evidence (unknown powder, shoe print, blood sample, cartridge case, etc) can be associated with either a group (class) or specific (individual) source. These associations may help investigators establish links between the crime scene, the parties involved, and/or the evidence. These are pretty simple concepts but even organizations like the National Academy of Sciences (NAS) can get confused about their meaning.
Class characteristics are intentional or recurring characteristics shared by one or more other objects in that group. For example, the outsole of a Nike Air Force II size 10 athletic shoe is manufactured to have a particular design. All size 10 Nike Air Force II outsoles manufactured from the same mould will have the same general appearance. It’s what distinguishes it from say a size 10 Reebok RealFlex athletic shoe. So if we find a shoe impression at a crime scene and can determine it’s a Nike Air Force II from the design elements it tells us the style of shoe but it won’t help us distinguish it from other Nike Air Force II athletic shoes of the same size.
Individual characteristics, on the other hand, are unintentional, random, and unique features of that object that distinguish it from other objects having the same class characteristics. Continuing with the footwear theme that would include specific cuts, scratches, abraded edges, foreign objects (nails), (not general wear) etc. that occur through the use and/or abuse of the footwear during its lifetime. Examiners look at the location, orientation, size, and shape of these defects to help distinguish one Nike Air Force II from another. The more individual characteristics present, the greater the degree of association (or elimination) between the evidence (impression) and the known source (shoe).
So in practical terms, if you found a shoe impression at a crime scene that showed extensive wear on the heel but the suspect shoes looked brand new then you could say that the suspect shoe did not make the impression; even though both may be of the same size, style, and manufacturer. That is not to say that class characteristics are not valuable. I won’t get into the concept of combined class characteristics here but I have seen suspects confess to crimes after being shown class character evidence i.e. “we found this Nike shoe impression at the crime scene and you’re wearing the same style Nike shoe”. Obviously they, like the NAS, don’t entirely understand the difference.
Posted on May 18, 2011, in General, The Crime Laboratory, The Crime Scene and tagged class characteristic, crime, Crime Scene, csi, detective, fiction, footwear evidence, forensics, individual characteristic, murder, mystery, police, thriller, tom adair. Bookmark the permalink. 15 Comments.