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Chapter 3 Physical Evidence.

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Presentation on theme: "Chapter 3 Physical Evidence."— Presentation transcript:

1 Chapter 3 Physical Evidence

2 Overview Types of Physical Evidence
Difference between Identification & Comparison of physical evidence Individual and Class characteristics of physical evidence National Databases Reconstructing the Crime Scene

3 Physical Evidence Almost anything can be Physical Evidence.
The purpose of recognizing physical evidence is so that it can be collected and analyzed. It is difficult to ascertain the weight a given piece of evidence will have in a case as ultimately the weight will be decided by a jury.

4 Common Types of Physical Evidence
Blood, Semen, Saliva Documents Drugs Explosives Fibers Fingerprints Firearms & ammunition Glass Hair Impressions Organs & physiological fluids Paint Petroleum products Plastic bags Plastic, rubber, & other polymers Powder residues Serial numbers Soil & minerals Tool marks Vehicle lights Wood & other vegetative matter

5 Identification The process of determining a substance’s physical or chemical identity with near absolute certainty Testing procedures that give characteristic results for specific standard materials Used repeatedly to prove the identity of suspect materials Number and type of tests needed to identify a substance sufficient to exclude all other substances

6 Comparison Two step process
The process of ascertaining whether two or more objects have a common origin Subjects a suspect specimen and standard/reference sample to same tests Two step process Combinations of select properties are chosen from the suspect and standard/reference specimen for comparison Forensic scientist must draw conclusions about the origins of the specimens

7 Individual Characteristics
Properties of evidence that can be attributed to a common source with an extremely high degree of certainty In all cases, it is not possible to state with mathematical exactness the probability that the specimens are of common origin. It can only be concluded that this probability is so high as to defy mathematical calculations or human comprehension.

8 Class Characteristics
Properties of evidence that can be associated only with a group and never a single source Here again, probability is a determining factor. Nevertheless, the high diversity of class evidence in our environment makes their comparison very significant in the context of a criminal investigation.

9 Class Characteristics
Most items of physical evidence retrieved at crime scenes cannot be linked definitively to a single person or object The value of class physical evidence lies in its ability to provide corroboration of events with data that are, as nearly as possible, free of human error and bias.

10 Individual or Class? the matching ridge characteristics of two fingerprints comparison of random striation markings on bullets / tool marks the comparison of two single-layer automobile paint chips of a similar color the comparison of irregular and random wear patterns in tire or footwear impressions Comparison of mass produced synthetic fiber the comparison of two blood specimens for blood type the comparison of handwriting characteristics the fitting together of the irregular edges of broken objects in the manner of a jigsaw puzzle Comparison of commercial potting soil matching sequentially made plastic bags by striation marks running across the bags

11 Value of Physical Evidence
Lies in its ability to corroborate events with data in a manner that is, as nearly as possible, free of human error and bias As the number of different objects linking an individual to a crime scene increases, so does the likelihood or probability of that individual’s involvement with the crime A person may be exonerated or excluded from suspicion of physical evidence collected at a crime scene is found to be different from standard/references samples collected

12 Physical Evidence Significance
There are practical limits to the properties and characteristics the forensic scientist can select for comparison Natural Limits Modern analytical techniques have become so sophisticated and sensitive that natural variations in objects become almost infinite – no two things in this world are alike in every detail.

13 Forensic Databases IAFIS,
The Integrated Automated Fingerprint Identification System: a national fingerprint maintained by the FBI. CODIS, The Combined DNA Index System enables federal, state, and local crime laboratories to electronically compare DNA profiles. NIBIN, The National Integrated Ballistics Information Network allows firearm analysts to acquire, digitize, and compare markings made by a firearm on bullets and cartridge casings. PDQ, The International Forensic Automotive Paint Data Query database contains chemical / color information to original automotive paints. SICAR (shoeprint image capture and retrieval) shoeprint database.

14 Reconstruction The method used to support a likely sequence of events at a crime scene by observing and evaluating physical evidence and statements made by those involved with the incident 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

15 Reconstruction Physical evidence left behind at a crime scene plays a crucial role in reconstructing the events that took place surrounding the crime. 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

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