Today, 9/18
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Transcript of Today, 9/18
Today, 9/18
1 – Short lecture (take out notes)
2 – Start project
Chapter Three: Evidence
Two broad types 1) testimonial (a.k.a. eye-witness) 2) physical
Which do you think has more reliability and why?
PHYSICAL EVIDENCE
Chapter 3
Physical Evidence Two major categories:
individual characteristics class characteristics
Individual characteristics: allow techs to attribute evidence to a common source with a high degree of certainty
Class characteristics: the properties of the evidence can only be associated with a certain group and never with a single source
Examples
Individual characteristics: two finger prints having same whorl
pattern broken pieces of glass that fit
together like a puzzle bullet striations
Examples continued
Class characteristics Blood type Tire prints that do not have wear
patterns yet Fiber found in most car fabric
Identification
Determine the substance via chemical or physical properties
Must be able to exclude all other substances
Might take one test or might need multiple tests
Identification
The purpose of identification is to determine the physical or chemical identity with as near absolute certainty as existing analytical techniques will permit.
1st - adoption of testing procedures that give characteristic results for specific standard materials.
Results have been established, they may be permanently recorded and used repeatedly to prove the identity of suspect materials.
2nd - the #and type of tests needed to identify a substance be sufficient to exclude all other substances.
Common Types of Identification
Chemical composition of an illicit drug. Gasoline in residues recovered from the
debris of a fire, or it may have to identify the nature of explosive residues—for example, dynamite or TNT.
Blood, semen, hair, or wood are also very common and, as a matter of routine, include a determination for species origin.
Comparison
Uses standard/reference samples Determines common origin of
evidence
Crossing Over Crossing over the line from class to
individual does not end the discussions. How many striations are necessary to
individualize a mark to a single tool and no other? How many color layers individualize a paint chip
to a single car? How many ridge characteristics individualize a
fingerprint? How many handwriting characteristics tie a
person to a signature? These are all questions that defy simple
answers and are the basis of arguments.
Types of physical evidence
Blood, semen, saliva: found on clothes, cigarette butts etc
Documents: includes paper, ink, writing, charred or burned documents
Drugs: anything in violation of laws Explosives: anything with a charge
as well as residue from explosions
Hair Impressions: includes foot prints,
tire tracks and even tool markings Manufactured items: firearms,
ammunition, fibers, paint, glass, etc
Fibers: natural
Each major category of physical evidence will be a separate unit
Chapter Three activities will include evidence collection, probability calculations and using facial reproduction software.
Significance of Physical Evidence Do you think that there are tests for
every thing? Probability plays a major role in
significance Simply defined, probability is the
frequency of occurrence of an event. Chances are greater that the
evidence will only reveal class characteristics
Product Rule Determines frequency a certain
combination of characteristics occurs in a population
Multiply the frequency of each individual characteristic
Blood factors, A 26%, EsD 85%, PGM 2%. .26 x .85 x .02 = .44 percent, so less then 1 in 200 people.
Value of Physical Evidence
Class physical evidence has the ability to corroborate events.
The value/significance accorded physical evidence is left entirely to the jury of usually lay people.
Forensic Databases
• The Integrated Automated Fingerprint Identification System (IAFIS), a national fingerprint and criminal history system maintained by the FBI.
• TheCombined DNA Index System (CODIS) enables federal, state, and local crime laboratories to electronically exchange and compare DNA profiles.
• The National Integrated Ballistics Information Network (NIBIN) allows firearm analysts to acquire, digitize, and compare markings made by a firearm on bullets and cartridge casings.
• The International Forensic Automotive Paint Data Query (PDQ) database contains chemical and color information pertaining to original automotive paints.
• SICAR (shoeprint image capture and retrieval) is a shoeprint database.
Reconstruction
Supports likely sequence of events of the crime
Must contain observation, evaluated physical evidence and witness statements
Done with cooperation between police, criminalists and medical examiner
Figure 3–9 Crime-scene reconstruction relies on the combined efforts of medical examiners, criminalists, and law enforcement personnel to recover physical evidence and to sort out the events surrounding the occurrence of a crime.
The Role of Physical Evidence The physical evidence left behind at a crime
scene plays a crucial role in reconstructing the events that took place surrounding the crime.
Although the evidence alone does not describe everything that happened, it can support or contradict accounts given by witnesses and/or suspects.
Information obtained from physical evidence can also generate leads and confirm the reconstruction of a crime to a jury.
The collection and documentation of physical evidence is the foundation of a reconstruction.
Figure 3–8 A laser beam is used to determine the search area for the position of a shooter who has fired a bullet through a window and wounded a victim. The bullet path is determined by lining up the victim’s bullet wound with the bullet hole present in the glass pane.