Chapter 3 Physical Evidence

Identification Determine physical or chemical identity of a substance w/ as near absolute certainty 2 steps Adoption of testing procedures that give characteristic results for specific standard materials Once test results been established, theymay be permanently recorded and used repeatedly to prove ID of suspect materials ID requires that the # and type of tests needed to ID a substance be sufficient to exclude all other substances Devise a specific analytical scheme that will eliminate all but one substance from consideration Each type of evidence requires different test, and each test has a degree of specificity

Comparison Comparison analysis subjects a suspect specimen and a standard specimen to same tests to determine if they have common origin 2 steps: Combination of select properties are chosen from the suspect and the standard for comparison Forensic scientist must be prepared to render a conclusion with respect to the source Narrows the evidence down to class or individual characteristics

Individual Characteristics Evidence that can be associated with a common source with an extremely high degree of probability EX – matching ridge charac of 2 fingerprints, striation markings on bullets Not possible to state with mathematical exactness the probability that the specimens are of common origin; only be concluded that the probability is so high that it defies mathematical calculations

Class Characteristics Properties of evidence that can only be associated with a group and never with a single source Limitations of science Frequent inability of lab to relate physical evidence to common origin w/ high degree of certainty EX – 2 1-layer auto paint chips = might be able to narrow down to a specific model at best 2 7-layer paint chips = much higher degree of certainty to one specific car

Weaknesses of Evidence Inability of examiner to assign probability values to class level physical evidence Difficult to determine probability especially with mass produced products It’s an endeavor of forensic science to create and update statistical databases to evaluate significance of class physical evidence Most items of physical evidence only specific to class level of characteristics

All in the presentation Value of class physical evidence lies in it ability to provide corroboration of events with data that are free of human error and bias Weight and significance accorded physical evidence is left to trier of the fact Lawyers present information in specific ways to emphasize the big picture to the jury Physical evidence can exclude or exonerate a person from suspicion

Debate amongst the scientists Where is the line drawn for the level of certainty for specific evidence to go from class characteristics to individual? How many striations are necessary to individualize a mark to specific tool? How many paint layers are needed to specify one specific car instead of just a model?

Crime-scene reconstruction Forensic scientists also participate in effort to reconstruct the events that occur prior to, during, and after the crime Reconstruction is collaborative effort that includes law enforcement, medical examiners, and criminalists Def’n – method used to support a likely sequence of events by the observation and evaluation of physical evidence and statements

reconstruction Evidence alone doesn’t describe everything; can support or contradict accounts given by witnesses Evidence can also generate leads and confirm the reconstruction of a crime to a jury

Remember processing? 1st – protect/secure crime scene so no evidence is called into question for contamination Initial walk-thru – observe and hypothesize the events; document observations Collect physical evidence, record all observations through photo, sketch, and notes

Medical examiner and criminalist Positioning of victim can reveal pertinent info Trained ME examine victim to see if been moved Use a laser beam to plot the approximate bullet path in trajectory analysis = position of shooter and victim Locate GSR on clothes = approx distance betw shooter and victim