Physical Evidence.

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Presentation transcript:

Physical Evidence

Trace fibers/ prints from suspects Discarded items or Clothing or gloves tools Broken glass Broken glass (inside/outside) often causes cuts/blood Latent prints Weapons (fingers/palm)

Significance of physical evidence Identification test - Determine physical or chemical identity Eg. Accelerant residues in fire debris, drug mixtures. Any test must exclude all other substances. Conclusions have to be substantiated beyond all reasonable doubt. 1.Physical evidence can be either identified or compared. 2.identification with as near absolute certainty as exsisting techniques allow. 3. Each test has a differnet degree of specificity too!

Comparison test - determine within the same test whether specimens have a common origin. eg. Hair, paint chips. Probability defines the mathematical odds or frequency of the specimens being of the same origin.

Individual versus Class Characteristics. Individual: properties of evidence can be attributed to a common source with a high degree of probability. Class: properties of evidence that can only be associated with a group and never with a single source.

Which are individual and which are class characteristics? 1. Fingerprints from 2 different sources 2. Two paint chips with one layer of paint 3. Two paint chips with several layers of paint 4. Markings on tools or bullets 5. Two blood specimens of human origin typed as “O positive”.

Class physical evidence - Allows corroboration of events with data that are free of human error and bias. - it can exonerate or exclude a person from suspicion. - further testing may allow class physical evidence to have individual characteristics.

Blood Typing Blood Factors Frequency O 37% Rhesus + 46% EsD1 85% PGM 2+2- 2% Still class characteristics//multiply frequencies to obtain % of probable suspects. BUT, DNA profile would be best//DNA=individual characteristic

Crime scene reconstruction

Crime scene reconstruction Team effort! Reconstruction: likely sequence of events by observation and evaluation of the physical evidence AND statements made by witnesses and those involved with the incident. Collaborative between law enforcement personnel, medical examiners and forensic scientists.