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Examination of Physical Evidence: Identification vs. Comparison

Explore the fundamental processes of identifying and comparing physical evidence, from determining substance identity to ascertaining common origins of objects. Learn key characteristics, mathematical probabilities, and the impact of Locard's Principle of Exchange.

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Examination of Physical Evidence: Identification vs. Comparison

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  1. Examination of Physical Evidence

  2. Identification vs. Comparison • Identification: The process of determining a substance’s physical or chemical identity • ex. heroin, gasoline, blood, hair (what species?), etc. • There must be tests that give repeatable results for each item. • May be one step, may be 5 or 6 steps.

  3. Comparison • The process of ascertaining whether two or more objects have a common origin. • ex. Similarities in hair found at a crime scene to a suspect’s hair; similarity in paint chips to paint from the suspect’s car.

  4. Comparison • First, you have to have a list of things to compare—a series of standard properties. • ex. Hair – look at the cuticle, cortex, and medulla and compare to one from a suspect

  5. Comparison • Second, use that list to compare—do they come from the same source? • Even if they’re indistinguishable, are they definitely from the same source? • NO

  6. Quality & Usefulness of Physical Evidence • Class Characteristics • Individual Characteristics • Mathematical Probability • Rarity • Locard’s Principle of Exchange

  7. Individual versus Class Evidence

  8. Individual versus Class Evidence

  9. Individual versus Class Evidence

  10. Individual versus Class Evidence

  11. Individual versus Class Evidence

  12. Collective Presence • May lead to an extremely high certainty that they originated from the same source. As the number of different objects link an individual to a crime increases, the probability of involvement increases dramatically.

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