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.
Boot outsole showing class characteristics
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).
Notice the cut across the coffin-shaped element
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.
Great information for me because shoes play an important role in my first mystery novel, a work in progress.
Thanks Violet, shoes can get pretty complicated depending on how detailed you want to get but if you have questions that come up during your process feel free to ask me. That is one of my areas of expertise. Tom
Great information really helpful in a homework assignement.
Glad to hear it. What class are you taking?
I am having trouble explaining the two class charateristics and Indiviual charateristics could you help me with using other examples to help me understand thank you:)
Sure, the Nike “Swoosh” symbol (element) is an example of a class characteristic. It is present on the outsole of many Nike shoes. An individual characteristic would be a cut (caused randomly during the normal use of the shoe) going through that Nike Swoosh symbol.You could gather 1,000 pairs of Nike shoes of the same make and size and none of them would have the same cut in the same position and orientation. Does that example help clear it up?
Is a bullet striation considered a class or individual characteristic?
It is an individual or unique characteristic made by the tools to finish the rifling in the barrel. There have been many studies of sequentially manufactured barrels showing that those striations do change from barrel to barrel.
Thanks, that was informative ! I have a question though..
If I had 2 pairs of the exact same shoe. Then I wore one for a year but left the other untouched. After that year would they make different shoe prints ?
Hi Nora, that’s a good question. If you wear the one pair of shoes on a variety of hard surfaces then the outsole will begin to wear down. Through the use and abuse of the shoe you will see small tears, cuts, divets, etc on the outsole that will distinguish it from the other pair of shoes. They will still retain the same class characteristics though. A simple way to think of it is this. Imagine you have two identical cars; one you drove everyday and one you kept covered in the garage. After a year the one you drive will probably have a number of small randomly placed paint chips and door dings or other damage that will make it distinguishable from the car in the garage. Does that make sense?
Yes thank you
Besides the general appearence, what are the class characteristics of the Nike air force II size 10 shoe?
The class characteristics are essentially the general appearance of the outsole but do a Google image search for “Nike Air Force 2″ and you can see a photo of the outsole to help you see some of the elements. Kind of hard to describe in words alone
So say the person put more pressure on the heel and it shoes in the imprint would that be a class characteristic?
No. A class characteristic is a manufacturing characteristic that is shared by all shoes made with that mold. What you are describing is a feature of gait or pathology (injury) that may be shared by many people but not all. It may be helpful to rule out a suspect or include them in a suspect pool but not for identification. Hope that helps. Tom