In general, all forms of evidence have class or individual characteristics. Class characteristics are not unique to a particular object but place the particular bit of evidence into a group of objects. Individual characteristics narrow down the evidence to a single, individual source.
The type of handgun with which a victim is shot is a class characteristic. For example, if the bullet came from a .38 caliber handgun, every .38 caliber handgun on the planet is the possible murder weapon. However, finding a suspect's fingerprint (an individual characteristic) on a .38 caliber handgun suggests that this .38, to the exclusion of all others, was the murder weapon. This is particularly true if the killing bullet can also be matched to this particular .38.
Alternatively, blood recovered from a crime scene that tests show is type B (a class characteristic) could have come from any of the tens of millions of people who share this blood type. If the suspect has type B blood, he remains a suspect. From there, DNA (an individual characteristic) from the suspect and DNA from the blood evidence are tested to determine conclusively whether they match. If, however, the suspect's blood is type A, he then is excluded as the source of the blood.
A single piece of class evidence rarely can be used to convict someone, but it can be and often is used to exonerate someone. However, when multiple types of class evidence associate one suspect with the crime and crime scene, the weight of that evidence can make for a stronger case, which is what happened to Wayne Williams.
In December 1981, Williams was tried for the Atlanta Child Murders based largely on class fiber evidence. Multiple fibers, 28 different types in all, were found on several of the victims. These fibers chemically and optically matched fibers taken from Williams's home and cars. Blue, yellow, white, and yellow-green fibers of various synthetic types were similar to fibers taken from Williams's kitchen and backroom carpets, bedspread, throw rug, and car liner. Hairs matching those of his dog also were found. Williams was convicted.
In the case against Wayne Williams, the sheer number of the pieces of class evidence made coincidence extremely unlikely. The odds that someone else left behind that particular combination of fibers and hair are astronomical. Although class evidence isn't absolute proof that a suspect is connected with a particular location and each bit of class evidence taken separately may not be strong, a large number of matching pieces of class evidence significantly boosts the probability that a suspect was present at the crime scene. A handful of class evidence is statistically equal to a single fingerprint.
The most individualizing types of evidence are fingerprints and DNA, because they're pretty much like snowflakes in that no two people have the same prints or, with the exception of identical twins, the same DNA. Impression evidence such as marks left on a fired bullet, shoeprints, tire tracks, and toolmarks may be unique and therefore have individual characteristics.
Fracture or tear patterns can be individualizing; the way glass breaks, paper tears, and cloth rips is unique in each situation. When a piece of glass taken from a crime scene and a piece of glass removed from a suspect's broken headlight fit neatly together like the pieces of a jigsaw puzzle, the pieces share a common source.
The overriding principle in analyzing individual characteristics is that no two things are exactly alike. No two guns mark a bullet the same way. No two pieces of glass fracture in the same manner. No two pairs of shoes or sets of car tires wear in exactly the same way.