THE CHARACTERISTICS OF PHYSICAL EVIDENCE
What we’re talking about here are two basic and very different descriptors: class characteristics and individual characteristics.
It’s important to note that all physical objects possess both class and individual characteristics; and it’s very important that the two not be confused!
A collection of hammers illustrates "class characteristics." Credit: USFWS
Class Characteristics … are physical qualities shared by a group of ‘like’ items, such as:
- A collection of miscellaneous hammers
- the chemical structure of heroin molecules
- the tread patterns on a brand of new athletic shoes
- the species source of a blood sample
- the ABO typing of a blood sample (ie: AB negative)
- the weight and dimensions of a new, unfired cartridge
- the make and model of a firearm
A fingerprint pattern illustrates "individual characteristics." Credit: USFWS
Individual Characteristics … are physical qualities that are unique to an individual evidence item, such as:
- a fingerprint pattern
- a pattern of cuts and wear marks on the tread patterns on a used pair of athletic shoes
- the resulting STR (CODIS) DNA code for a
blood or semen sample
- the striation marks on a fired bullet
- the pattern of ejector, extractor, breech and
firing-pin marks on a fired cartridge
And finally, let's look at some important factors that must be considered when evidence is collected from a crime scene.
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