Transcript Chapter 3 Physical Evidence

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.
2.
3.
4.
5.
6.
7.
Blood, semen, saliva
Documents
Drugs
Explosives
Fibers
Fingerprints
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
Identification
 Process
of determining a substance’s
physical or chemical identity.
 Drug
analysis
 Species determination: human or not
 Explosive residue analysis
Identification
Steps
Involved:
Design
systematic analysis that
will always test for that substance
Testing must eliminate all other
possibilities
 Some substances require 1-10 tests

FS
must be prepared to render a
conclusion with respect to the
origin of the specimen
Comparison
 Attempts
to ascertain whether two or
more objects have a common origin.
 Use properties of suspect and control.
 Gives conclusions as probability
 Two 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 analysis
 Put in team folders
The Product Rule (Probability)
Multiplying
together the
frequencies of independently
occurring events
Rolling 2 dices and getting
sixes
1/6
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 room
 12x5’ room
 Total sales divided between 10 states
 SO
82 rooms with this carpet in Georgia
 Product
rule (probability) =1 : 7,792
 Very low chance
Crime scene reconstruction
 The
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 CSR
 Medical
examiner
 Law Enforcement personnel,
 Criminalists
Finis’