Similar class characteristics, or the lack thereof, can only exclude a suspect, and should not be given an inordinate amount of weight on their own.
The largest area of observer-based disciplines is that of pattern evidence. Pattern evidence includes fields such as firearm and toolmark examination, handwriting analysis and forensic odontology.
Analysis of pattern evidence can be highly subjective, relying heavily on human interpretation. A scientist analyzing tool marks, fingerprints or firearms develops a discerning eye only through rigorous training that involves years of experience and working on hundreds of cases. It is also useful to understand that human beings are very good at pattern recognition and comparison.
When a peer reviews a completed pattern analysis—or when a jurist tries to grasp how the scientist arrived at his or her conclusion, the subjective nature of the analysis might cause others to disagree with the findings.
But a conscientious analyst can take certain steps to give that peer or jurist a better chance at following the thought process. For instance, a fingerprint examiner can mark what is considered common minutiae in the latent and known samples, or describe the common minutiae and their position in the images through detailed lab notes.
The characteristics of pattern evidence fall into one of two categories:
Class characteristics: those that are common to a group of objects or people
In one potential scenario, tire treads are found in the dirt at a crime scene. The forensic analyst identifies the brand and style of tire. While the suspect’s car has those tires, so do tens of thousands of other cars. That is a comparison to a class of characteristics – the physical qualities shared by a group of like objects. Stating that the sample was a match to a particular tire or car would be a scientifically invalid statement.
Individual characteristics: Those that are common to one specific object or person
In that same scenario, if the pattern in the dirt was left by a sneaker with worn edges and cracks in the sole, and shared several points of similarity with a shoe in the suspect’s closet, then this comparison would be between individual characteristics. The impression is consistent with physical qualities unique to a specific object or person, and should be given significantly more weight than class characteristics.
That said, some scientists do not agree on what qualifies as an individual characteristic. Some may maintain that the wear pattern found at the scene might be the same for people with the same shoes and same build, who gave their shoes the same amount of wear.
Additionally, a ‘combination of characteristics’ may be individualistic even if the individual characteristics are not. A certain amount of characteristics occurring together could likely prove individuality, however, without population statistics to back that up, there is no way of deciding how many characteristics are needed, or if one characteristic is more valuable or probative than another.
In the next section, learn more about fingerprint evidence, one of the earliest and most highly respected forensic disciplines.