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Chapter 3 Continued. Criminal Investigations/O’Connor. The Significance of Physical Evidence. Forensic Science is unable to assign an exact or even approximate probability values to the comparison of most class physical evidence.
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Chapter 3 Continued Criminal Investigations/O’Connor
The Significance of Physical Evidence • Forensic Science is unable to assign an exact or even approximate probability values to the comparison of most class physical evidence. • As more things in our world are mass produced, this becomes harder and harder.
Occasionally statistical data is available for use; like population frequency of blood factors. • More often forensic scientists must rely on personal experience when interpreting the significance of class physical data
Most items of physical evidence retrieved at a crime scene cannot be linked definitively to a single person or object. • Investigators try to find evidence with individual characteristics, like fingerprints- the chances of finding such evidence is much lower than finding class evidence.
Criminal cases are built for the courtroom around a collection of diverse elements, each pointing to the guilt or involvement of a party in a criminal act. Often, most of the evidence gathered is subjective, prone to human error & bias. The believability of eyewitness accounts, confessions, & informant testimony can all be disputed, maligned, & subjected to severe attack & skepticism in the courtroom. In such cases errors in human judgement are often magnified to detract from the credibility of the witness.
Assessing the Value of Evidence • Defining the significance of an item of class evidence in exact mathematical terms is usually a difficult if not impossible goal. As the number of different objects linking an individual to a crime increases, the probability of involvement increases dramatically.
A person may be exonerated or excluded from suspicion if physical evidence collected at a crime scene is found to be different from standard/reference samples collected from that subject.
Forensic Databases • Computer technology has dramatically altered the role of the crime laboratory in the investigative process. • The creation of computerized databases that not only link all fifty states, but tie together police agencies throughout the world.
Fingerprint Databases • Integrated Automated Fingerprint Identification System (IAFIS) ~ maintained by the FBI since 1999. • Nearly 50 million subjects, which are submitted voluntarily to the FBI.
DNA Databases • Combined DNA Index System (CODIS) • CODIS creates investigative leads from two sources: ~The forensic index-110,000 profiles of unsolved crime-scene evidence. & ~The offender index- 3 million convicted or arrested individuals.
Other Databases • National Integrated Ballistics Information Network (NIBIN) • International Forensic Automotive Paint Data Query (PDQ)
Crime-Scene 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.
Crime-scene reconstruction relies on the combined efforts of medical examiners, criminalistics, and law enforcement personnel to recover physical evidence and to sort out the events surrounding the occurrence of a crime
Examples of crime scene reconstruction include determining whether a body was moved after death, determining whether a victim was clothed after death, analyzing bullet trajectory, analyzing blood spatter, determining the direction from which projectiles penetrated glass objects, estimating the distance of a shooter from a target, and locating primer residue on suspects.
