Ch 3 - Physical Evidence Forensic Science Class Characteristic Properties of evidence associated with a group and never a single source Comparison Ascertaining if two or more objects have a single origin Individual Characteristic Properties of evidence that can be attributed to a common source with a high degree of certainty Types of Evidence Blood, Semen, Saliva - analyze to determine identity Organs and Physiological Fluids - tested to detect presence of drugs/poisons Documents - paper, ink, burned/charred documents Drugs - any illegal substance Explosives Fibers Fingerprints - latent and visible Firearms and Ammunition Paint - often collected in hit and run cases Petroleum Products - gasoline, grease, or oil Plastic Bags - garbage bags Plastic, Rubber, and other Polymers Powder Residue - GSR Serial Numbers - used in stolen property cases Glass Hair - animal or human Impressions - tire marks, footprints, glove/fabric imprints, bite marks Soil and Minerals - link a person to a location Tool Marks Vehicle Lights - examined to determine if lights were on or of at time of impact
Examination of Physical Evidence - By Means of Identification Identification Determine physical or chemical identity Crime labs commonly identify unknown powder substances as heroine, cocaine, or barbiturates Also establish origins for blood, semen, hair, and wood Identification Process first requires a procedure that gives characteristic results for a standard substance (i.e. given a known sample of cocaine, the test must prove the know sample if cocaine) Second, the test must prove the substance s identity exclusive of all other options (i.e. Scientist knows identity after testing, beyond any reasonable doubt in a court of law) Examination of Physical Evidence - By Means of Comparison analysis subjects both the suspect specimen and a standard sample to the same tests to determine whether they have a common origin i.e. - Suspect may be linked to a crime scene by analyzing hairs found at the crime scene to hairs from suspects head Comparison Two step procedure, first combinations of select properties are chosen from the suspect and standard sample for comparison (i.e. paint may have density, color, and composition tested for a match) The forensic scientist must secondly determine from the results if a match is found If one or more properties don t match, then the origin is not common If they do match, it is certain that they did come form the same origin? Not always... Must take into account the probability factor (the odds that a certain event will occur, like flipping a coin 100 times and seeing how many outcomes are heads and tails)
Comparison - Individual Characteristic Evidence associated with a single source with an extremely high degree of probability Examples may be matching the ridge characteristics of fingerprints, matching random striations from bullets, tire and footwear impressions Comparison - Class Characteristic Evidence that can be associated with a group and never a single source Probability is again the determining factor i.e. comparing a single layer paint chip from two cars may not show a common source, but two paint chips with several layers each may prove a common source The single layer chip is a class characteristic since it can establish a color of a certain model car, but there may be thousands of that particular car model The several layer chip may show individual characteristics Fingerprint Probabilities French scientist Victor Balthazard determined the mathematical probability of having two identical fingerprints as one in 1x10 60!!! This chance is so minuscule that it is understood to be a zero percent chance Probability and Product Rule Intro Case 1 Case 2 Case 3 Blond Hair - 32% Red Hair - 11% Brown Hair - 51% Type O - 43% Type B - 12% Type AB - 3% Arch Fingerprints - 5% Loop Fingerprints - 65% Whorl Fingerprints - 33% In which case does the evidence most strongly suggest that the suspect is guilty? % s are used for examples only, NOT scientifically accurate for all you smarties out there!
Product Rule Multiplying the frequency of each factor together will determine how common factor combinations are in the general population i.e. analyzing a blood sample to be type A is not individualized enough for a high degree of certainty But taking into account both EsD and PGM factors, the chances for a individual characteristic increase greatly...(see next slide) O.J. Simpson Crime Scene Blood Factors Blood Factors Frequency in the Population Type A 26% EsD 85% PGM (prothrombin gene mutation) 2% 26% x 85% x 2% = 0.44% or 1 in 200 people with this probability Factors did not match Nicole Simpson or Ron Goldman (two victims) but did match O.J. The forensic scientist did not feel this was enough evidence to show an individual characteristic though Significance of Physical Evidence Unlike CSI and Law and Order, reality is that most items of physical evidence cannot definitively be linked to a single person or object Investigators try to discover evidence with individual characteristics (fingerprints, tool marks, bite marks, bullets,...) but the chances of finding class physical evidence are far greater Cautions and Limitations in Dealing with Phys. Evidence The value of evidence relies on it being free of human error and bias But the determination of the significance for each piece of evidence relies on the trier (usually the jury of laypeople)
Goal of the Forensic Scientist How many striations are necessary to individualize a tool mark? How many layer matches are required to individualize a paint chip from a car? How many arches are need to match a fingerprint? The task of a forensic scientist is to identify as many characteristics, markers, and comparisons for each case as possible