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

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.

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

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  1. 2 Methods used to identify materials found at the scene of a crime

  2. Identification • Requires: adoption of testing techniques for specific materials. Must be able to be duplicated in subsequent tests. • Must EXCLUDE all other substances.

  3. 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.

  4. Individual characteristics

  5. Matching striations (markings)

  6. Wear patterns

  7. Fitting irregular pieces together

  8. 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. 

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

  10. 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).

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

  12. 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

  13. 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.

  14. 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

  15. Weakness of Evidence with Class Characteristics • Generally, indistinguishable from other, similar items of same manufacture • Little statistical data exist

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