1. Physical Evidence Chapter 3: Physical Evidence
Examples, Identification, Comparison, Limitations, Value and Relevance

2. Physical Evidence
Physical evidence is any and all objects that can establish that a crime has been committed or can provide a link between a crime and its victim or a crime and its perpetrator.

3. Physical Evidence

4. Common Types of Physical Evidence
Biological: Blood, semen, and saliva
Documents: handwriting, charred documents
Drugs: Any substance seized in violation of the laws regulating the sale, manufacture, distribution and use.
Explosives: Any device containing an explosive charge, as well as all objects removed from the scene of an explosion that may contain residues.
Fibers: natural or synthetic
Fingerprints: both latent (invisible) and visible
Firearms and ammunition: any firearm, discharged or intact ammunition.

5. Common Types of Physical Evidence
Glass
Hair
Impressions
Organs and physiological fluids
Paint
Petroleum Products
Plastic bags

6. Common Types of Physical Evidence
Plastic, rubber other polymers
Powder residues
Serial Numbers
Soil and Minerals
Tool Marks
Vehicle Lights
Wood and Other Vegetative Matter

7. Examination of Physical Evidence
The examination of physical evidence by a forensic scientist is usually undertaken for identification or comparison.

8. Identification
The process of determining a substance’s physical or chemical identity.
The crime laboratory is frequently requested to identify the chemical composition of an unknown substance.

9. Examples of Identification
Identify the chemical composition of an illicit drug preparation that may contain heroin, cocaine, barbiturates, and so on.
Identify gasoline in residue recovered from the debris of a fire
Identify the nature of explosive residues, example: dynamite or TNT.
Identification of blood, semen, saliva, hair and include determination for species origin.

10. Identification
The process of identification first requires the adoption of testing procedures that give characteristic results for specific standard materials.
Once these test results have been established, they may be permanently recorded and used repeatedly to prove the identity of suspect materials.

11. Identification
Second, Identification requires that the number and type of tests needed to identify a substance be sufficient to exclude all other substances.

12. Identification

13. Identification – Putting it Together
A white powder gets submitted as physical evidence to a crime lab.
The forensic scientist must use standardized tests with known results to try to determine the chemical identity of the substance.
Suppose the white powder is identified as heroin.
The forensic scientist must then devise a specific analytical scheme that will eliminate all other possibilities of that substance being anything but heroin.
The forensic scientist’s test results must have been comprehensive enough to have excluded all other drugs or, for that matter all other substances from consideration.

14. Identification – Putting it Together

15. Identification – White Powder?
An officer in the field can use a simple test to see if the white powder might be heroin.
The white powder is submitted to a forensic laboratory.
The forensic examiner then subjects the white powder to a series of preliminary standardized tests that have known results to try to confirm the identity of the substance.
Once a possible identity is found then the forensic examiner MUST perform a battery of tests to “prove” or “confirm” that all other substances have been eliminated.

16. Problems and Pitfalls
The forensic scientist has little or no control over the quality and quantity of the specimens received.
Simple rules can not be devised for defining what constitutes a thorough and foolproof analytical scheme.
Each type of evidence requires different tests and each test has a different degree of specification.
It is left to the forensic scientist to determine at what point the analysis used in identification can be concluded and the criteria for positive identification satisfied.

17. Comparison
A comparison analysis subjects a suspect specimen and a standard/reference specimen to the same tests and examinations for the ultimate purpose of determining whether or not they have a common origin.
The process of ascertaining whether two or more objects have a common origin.

18. Comparison: Examples
Comparison of a hair follicle found at the crime scene with a standard/reference hair from a suspect.
Paint chip found on a hit and run victim’s garment compared to a paint chip removed from a suspected vehicle

19. Comparison: Two Step Procedure
First combinations of select properties are chosen from the crime-scene specimen and the standard-reference specimen for comparison.
Second- the forensic scientist must be prepared to render a conclusion with respect to the origins of both specimens. Do they or do they not come from the same source?

20. Comparison - Possibilities
If one or more properties or characteristics do NOT agree between the crime-scene specimen and the standard/reference specimen, than the examiner will state –”they are not of the same source or origin”.
But What (if)
If all the properties compare and the specimens, as far as the examiner can determine are indistinguishable. Does it logically follow that they come from the same source??? NOT NECESSARILY so. It depends upon: probability, individual characteristics and class characteristics.

21. Probability Probability is the likelihood that an event will occur.
Probability defines the odds at which a certain event will occur.
The greater the number of similarities known as individual characteristics between the crime-scene specimen and the standard/reference specimen, the higher the probability that the objects have a similar or common origin.

22. Individual Characteristics
Properties or characteristics of evidence that can be attributed to a common source with an extremely high degree of certainty.

23. Individual Characteristics - Examples
Matching ridge characteristics of two fingerprints.
The comparison of random striation markings on bullets or tool marks.
The comparison of irregular and random wear patterns in tire or footwear impressions.
The comparison of handwriting characteristics
The fitting together of the irregular edges of broken objects in the manner of a jigsaw puzzle.

24. Examples of Individual Characteristics

25. Individual Characteristics

26. Probability and Individual Characteristics
Evidence that can be associated with a common source with an extremely high degree of probability is said to possess individual characteristics.
In all of the prior examples of individual characteristics: ridge patterns between fingerprints, tool markings, comparisons of wear patterns for tires, an exact probability of the two specimens being of common origin is not able to be calculated. It can only be concluded that this probability is so high as to defy mathematical calculations or human comprehension.
Example: The probability of two humans or even two fingers having the same fingerprints is 1 x 1060

27. Class Characteristics
Properties of evidence that can only be associated with a group and never with a single source.

28. Class Characteristics and Probability - Examples
Comparison of two chips of paint. One from the crime-scene and one from suspected vehicle. If each paint specimen is only one – layer thick the chance of their having originated from the same car is not nearly as great as if we compare two paint chips having seven similar layers, not all of which were part of the car’s original color.
The single layer paint chips have class characteristics since they could only be associated at best with a specific car model.
The seven layer paint chips have individual characteristics and have a high probability of originating from one specific car.

29. Class Characteristics and Probability Examples
A blood sample from a crime-scene and a standard/reference sample both are found to be Type A. Since 26% of the population is Type A then there is a high probability that they are not from a common origin or person.
BUT What if the crime-scene sample and standard/reference sample are found to have not only the type by three other proteins in common? With the increase in the number of blood factors in common, the probability is higher that they have a common origin.

30. Calculating Probability- Product Rule
Multiplying together the frequencies of independently occurring events to obtain the overall probability of occurrence for all events happening at once.
Example: Blood found at the scene of the Nicole Brown Simpson murder scene matched OJ Simpson’s blood with the following blood factors and frequency of occurrence: Type A 26%, EsD 85% and PGM2+2- 2%. When these are multiplied together, the overall probability that the blood is NOT OJ Simpsons is 0.44% or 1 in 200 that it was NOT OJ’s blood at the crime scene.

31. Individual Characteristics VS Class Characteristics
Evidence that can be associated with a common source with an extremely high degree of probability is said to possess individual characteristics.
Evidence associated with only a group is said to have class characteristics.

32. What is the greatest value of class physical evidence?
The value of class physical evidence lies in its ability to corroborate events with data in a manner that is, as nearly as possible, free of human error or bias.

33. What is the greatest weakness of class physical evidence?
The greatest weakness of class physical evidence is that it cannot relate physical evidence to a common origin with a high degree of certainty. Evidence possessing class characteristics can be associated only with a group and never with a single source.

34. Cautions and Limitations in Dealing with Physical Evidence
It is important to consider the relevance of scientific evidence before allowing it to be introduced into a criminal case because physical evidence is accorded great weight during jury deliberations. In addition, failure to take proper safeguards when determining the relevance of evidence may unfairly prejudice a case against the accused.

35. Why does a forensic scientist often opt not to use the most sensitive analytical techniques when comparing suspect substances?
A forensic scientist opts not to use the most sensitive analytical techniques when comparing suspect substances because, carried to the extreme, no two things in this world are alike in every detail. Measurements that are extremely sensitive may show differences between two samples that suggest samples came from different sources even though they actually came from the same source.