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 Evidence
Case 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 Characteristics
Generally, indistinguishable from other, similar items
of same manufacture
Little statistical data exist