2 methods used to identify materials found at the scene of a crime
TRANSCRIPT
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2 Methods used to identify materials found at the scene of a crime
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Identification
Requires: adoption of testing techniques for specific materials. Must be able to be duplicated in subsequent tests.
Must EXCLUDE all other substances.
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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.
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Individual characteristics
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Matching striations (markings)
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Wear patterns
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Fitting irregular pieces together
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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.
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Class Characteristics
Can be associated only with a group, not a particular source (paint used by a car manufacturer, for example).IDENTICAL!
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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).
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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.
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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
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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.
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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
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Weakness of Evidence with Class
CharacteristicsGenerally, indistinguishable from other, similar items of same manufacture
Little statistical data exist
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