1. Chapter 3 Physical Evidence

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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?

9. 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

10. 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

11. 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

16. 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