2 Methods used to identify materials found at the scene of a crime

Identification

Requires: adoption of testing techniques for specific materials. Must be able to be duplicated in subsequent tests.

Must EXCLUDE all other substances.

Comparison

Compares a suspect’s specimens to a standard.

All properties must compare.

Whether that comparison can put a suspect at the scene, or just make it possible that he was, is dependent on the uniqueness of the standard that the specimen is being compared to.

Usually, this is supportive of all other evidence; generally, materials that are not unique cannot definitely place a suspected person at the scene.

Individual characteristics

Matching striations (markings)

Wear patterns

Fitting irregular pieces together

Fitting material to a given set of objects by

matching features (tears, striations, etc.)Probability of multiple pieces being a match, even if the original object was not unique, bolsters the case against a suspect. 

Class Characteristics

Can be associated only with a group, not a particular source (paint used by a car manufacturer, for example).IDENTICAL!

Blood type

When other blood factors are added in, it narrows the potential number of suspects.

You multiply the frequency of all the factors to determine the probability of a match to a particular suspect (X out of a given number).

Other EvidenceCase 1 Case 2 Case 3

Blond hair (32%) Red hair (11%) Brown hair (51%)

Type O blood (43%) Type B blood (12%) Type AB blood (3%)

Arch Fingerprints (5%)

Loop Fingerprints (65%)

Whorl Fingerprints (33%)

Guess which Case is likeliest to have a higher degree of certainty in identifying the suspect.

Product Rule

Multiply the frequency of all the evidence

For example, 5% will give you 0.5 X the other frequencies

The product of those frequencies is the likelihood that two individuals would share the stated characteristics

Manufactured products have class characteristics.

Natural fibers and handmade items have more unique characteristics.

Fibers used to be able to be identified by dye lot – a unique characteristic that limited the possible fiber to a particular batch of manufacturer. Normally, today’s fibers are identical to all other fibers of that particular color.

Class Characteristics

Old-fashioned typewriters produced evidence that uniquely connected that document to a particular typist – how hard they typically struck the keys, how even their strike on the keys was (did they hunt & peck, did they touch type, how smoothly they typed).

Modern printers lack any individual characteristics

Weakness of Evidence with Class

CharacteristicsGenerally, indistinguishable from other, similar items of same manufacture

Little statistical data exist