Chapter 3 Physical Evidence
What is Physical Evidence?Any and all objects that can establish
that a crime has been committed
OR can provide a link between a crime and it’s victim or a crime and its perpetrator.
Common Types
1. Blood, semen, saliva
2. Documents
3. Drugs
4. Explosives
5. Fibers
6. Fingerprints
7. Firearms and ammunition
Common Types
8. Glass
9. Impressions
10. hair
11. Organs and Physiological fluids
12. Paint
13. Petroleum products
14. Plastic bags
15. Plastic, rubber, polymers
Common Types
16. Powder residues
17. Serial numbers
18. Soil and minerals
19. Tool marks
20. Vehicle lights
21. Wood and vegetative matter
IdentificationProcess of determining a substance’s
physical or chemical identity. Drug analysis Species determination: human or not Explosive residue analysis
IdentificationSteps Involved:
Design systematic analysis that will always test for that substanceTesting must eliminate all other
possibilitiesSome substances require 1-10 tests
FS must be prepared to render a conclusion with respect to the origin of the specimen
ComparisonAttempts to ascertain whether two or
more objects have a common origin.Use properties of suspect and control.Gives conclusions as probabilityTwo types of characteristics
Comparison:Two types of characteristics
Class characteristics: Substances can be associated with a group but
not individual source Blood types: use factors in blood A lot of these can ID suspects at a crime scene
Individual characteristics: Substances that are related at almost 100%
probability Fingerprints are 1x1060 that 2 peoples are the
same
Do Now 10/14:Get out Lab due today: hair analysisPut in team folders
The Product Rule (Probability)
Multiplying together the frequencies of independently occurring events
Rolling 2 dices and getting sixes1/6 x1/6= 1/36
The Product Rule (Probability)
O.J.’s bloodstain frequencies
Blood Factors Frequency
Type A blood 26%
EsD
estérase D
85%
PGM 2+2- 2%
Fiber Evidence and the Wayne Williams Trial Items from residence and station wagon.
Fiber Evidence and the Wayne Williams Trial
Tracking Carpet fiber between the two victims
Assuming: Carpet installed in one room12x5’ roomTotal sales divided between 10 states
SO 82 rooms with this carpet in GeorgiaProduct rule (probability) =1 : 7,792Very low chance
Crime scene reconstructionThe method used to support a likely
sequence of events by the observations and evaluation of physical evidence, as well as statements made by witnesses and those
involved with the incident.
Crime scene reconstruction
For CSRMedical examinerLaw Enforcement personnel,Criminalists
Finis’Finis’
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