The Distinction Between Individual and Class Evidence is Always Easy to Make
Aim: How can evidence be individualized? This lesson is part of a unit examining the probative value of different types of evidence.
Objectives of the Lesson:
Students will be able to:
1) distinguish between individual and class evidence.
2) determine the significance of class evidence.
Motivation:
Content of the Lesson:
Physical evidence is part of the "holy trinity" for solving crimes -- physical evidence, witnesses, and confessions. Without one of the first two, there is little chance of even finding a suspect. In homicide and sexual assault cases, physical evidence is the number one determinant of guilt or innocence. Physical evidence is also the number one provider of extraordinary clearances, where police can link different offenses at different times and places with the same offender. Working with physical evidence means being aware at all times of what the prosecutor needs to win the case in court.
"The best evidence is anything that can be linked to a unique, single, specific source. This is called individual evidence. Examples are fingerprints, handwriting, DNA patterns, and sometimes physical matches, such as a piece of broken glass that exactly fits to its mate (like a jigsaw puzzle piece). Unfortunately, most evidence is class evidence; this means that the object has characteristics common to a group of similar objects, not to one single object… A way to increase the probative value of class evidence is to find as many different types of objects as possible to link the suspect to the crime or the victim. (Probative means supplying proof or evidence.)" (Deslich and Funkhouser, 2006, p. 27)
Guided Practice for Application :
Activity: "Can This Evidence Be Individualized?" Adapted from Deslich and Funkhouser, 2006, p. 30-31.
Items are collected and set up around the classroom for students to examine while answering a series of questions using a worksheet. Background:
"Without doubt, the most important concept in criminalistics is identification, or what Paul Kirk called individualization (Kirk, 1936)… When a number of details are put together (as in points of comparison), so that they constitute a class of one (by itself), they are said to establish an identity, also called individual characteristics, or entities in a class by itself. At this point, if there are similarities between evidence from the crime scene and evidence from a suspect, the expert can say, without a doubt, that identity has been individualized.
"Identity is a set of characteristics (combinations of class characteristics or combinations of class and individual characteristics) by which a thing is recognizable or known. Identity is the same as pattern. A pattern is established, for example, when a particular piece of class evidence like fiber (for which there are large quantities in the population) is put together with another piece of class evidence like red hair (which usually only exists in the subpopulation of white people). It cannot be said the investigator has "individualized" anything at this point, but the red hair in this example can be considered enough of an individual characteristic which combines with the class characteristic to establish identity. At this point, if there is a "match" between the fibers and hair from the crime scene with a suspect, the examiner can say the crime scene (unknown or questioned) sample "may have come from the same source" as the suspect's (known or exemplar) sample, which is sufficient for probable cause. Identity can also be used to conclusively eliminate people as suspects.
"Class characteristics alone do not allow matches with a single suspect. Matches for evidence with individual characteristics do allow pinpointing a particular suspect. Matches at the individual level are called similarities, because in theory, there's no such thing as a perfect match. An example of a similarity, or match in individual characteristics, would be a number of comparable points of comparison between the friction ridge lines on a latent (crime scene) fingerprint with the fingerprints of a particular suspect. This kind of (true) match allows the examiner to say that, because the similarities outweigh any dissimilarities, the crime scene (unknown or questioned) sample "did come from the same source" as the suspect's (known or exemplar) sample, which is sufficient, most of time, for proof beyond a reasonable doubt" (O'Connor, 2004).
References:
Deslich, B., & Funkhouser, J. (2006). Forensic Science for High School. Dubuque, IA: Kendall/Hunt.
Kirk, P. (1936). The ontogeny of criminalistics. Journal of Criminal Law, Criminology, and Police Science, 54, 236-41.
O'Connor, T. (2004). An introduction to criminalistics and physical evidence. Retrieved on November 5, 2007 from http://faculty.ncwc.edu/TOCONNOR/315/315lect02.htm
Source: https://sites.google.com/site/crimesceneanalysis/class-vs-individual-evidence-2
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