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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