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Examination of Physical Evidence

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Presentation on theme: "Examination of Physical Evidence"— Presentation transcript:

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
Class Individual Definition Evidence that can only be associated with a group source and not a single source.  Evidence that can be associated with a single, common source with an extremely high degree of certainty. 

8 Individual versus Class Evidence
Class Individual Degree of certainty of common origin  Low High 

9 Individual versus Class Evidence
Class Individual Found at the Crime Scene Usually Sometimes

10 Individual versus Class Evidence
Class Individual Value Provide corroboration of events with data that are free from human error. Collective presence may lead to extreme high certainty. Individualizes suspect to a high probability.

11 Individual versus Class Evidence
Class Individual Examples Tire tracks Shoe impressions Blood type Paint layers Fibers DNA fingerprinting Teeth Fingerprints Bullets: the striation markings on bullets

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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