1. NATURE OF EVIDENCE Learning Objectives
Describe the difference between real and demonstrative evidence
Recognize and describe the difference between known and unknown evidence
Recognize and describe the difference between class and individual evidence
Give examples of positive and negative controls
Define false positive and false negative tests

2. INTRODUCTION Crime scene Investigation is a type of reconstruction.
Evidence consists of hints or pieces of data that help reconstruct a scene – in our case, a crime scene.
Evidence can be thought of as something that supports or rejects a theory about a crime, how it occurred, and who committed it.
Evidence is defined in law books as anything that would make an issue more or less likely that it would be without the evidence.
In other words, evidence is anything that tends to prove or disprove something at issue.

3. THE EVIDENCE IN COURT
How evidence is received in a court of law is extremely important to a crime scene professional
Because of the Constitution’s view of “innocent until proven guilty” and the rights of the accused, it is crucial that those in law enforcement understand the types of evidence, how it is perceived in court, how to testify to its significance, and the importance of proper evidence collection and handling.

4. POWER OF THE EVIDENCE IN COURT
How does one decide what is evidence and what isn’t?
Experience
Context
The type of crime guides the CSI in determining what may be evidence and should be collected.
Federal law states that evidence must be
Material
Probative
Material: Evidence is material if it pertains to the particular case that is being tried or investigated.
Probative: A piece of evidence that tends to prove or disprove a fact or assertion.

5. POWER OF THE EVIDENCE IN COURT CONTINUED
Probative Value
- Higher value if the evidence can prove something in court
- The lower the probability of an event, the higher the probative value
Probability – the frequency of the occurrence of an event
- Defines the odds that a certain event will occur (or the matching of a certain piece of evidence)
- Normally found by multiplying the odds
- Example: if a coin is flipped 100 times and comes up heads 50 times, the probability of coming up heads is 50 out of 100, or 5 out of 10

6. Frequency in Population (%) Decimal of the Frequency
POWER OF THE EVIDENCE IN COURT CONTINUED
Product Rule – calculate the overall frequency of occurrence in a population
Example: the OJ Simpson case: Blood factors from a crime scene that match OJ’s blood
Blood Factors
Frequency in Population (%)
Decimal of the Frequency A
26%
.26
EsD
85%
.85
PGM 2+2- 2%
.02
Product Rule x .85 x .02 = or 0.44%, or less than 1 out of 200 people would be expected to have this combination of blood factors

7. CLASSIFICATION OF EVIDENCE
Evidence may be categorized in a number of different ways. The class or type of evidence can be important in determining:
What value it has
How it should be collected
What else should be collected (controls, exemplars)
And most important, what conclusions can be drawn from a scientific examination of the evidence
There are also legal distinctions among different types of evidence that help determine its admissibility in court.

8. CLASSIFICATION OF EVIDENCE CONTINUED
The five major classification systems for evidence are:
Physical – Nonphysical
Real – Demonstrative
Known – Unknown
Individual – Class
Direct – Indirect

9. PHYSICAL – NONPHYSICAL EVIDENCE
Physical evidence consists of objects or things.
Nonphysical evidence is verbal testimony about a crime, or someone’s actions during a crime.
Example: If someone is seen running away from a bank robbery holding a bag of money, the action of running away is nonphysical evidence while the bag of money is physical evidence.

10. PHYSICAL – NONPHYSICAL EVIDENCE CONTINUED
Not every piece of evidence is an object.
Courtroom dramas often focus on motive – why someone would commit a crime. A motive is often required as an element of a crime.
What’s your opinion. Are the results of a polygraph test considered physical or nonphysical evidence?

11. REAL – DEMONSTRATIVE EVIDENCE
Real evidence is evidence generated by criminal activity. Real evidence:
Found at the crime scene (or elsewhere) and pertains to the crime
Examples: drugs, blood, bullets, fingerprints left at a crime scene or obtained from a suspect
Does not have to found at the crime scene, ex. A person is shot and killed. The weapon was not found at the crime scene but was found at the suspect’s house
The majority of evidence in criminal cases is real evidence.

12. REAL – DEMONSTRATIVE EVIDENCE CONTINUED
Demonstrative evidence is created to help explain or clarify real evidence.
Produced after the crime and not by the crime
Examples include sketches, videotapes, scale drawings, physical models of a crime scene, and computer reconstruction simulations of events such as fires, explosions, vehicle crashes, and structural failures
Case Study – flammable hair product

13. KNOWN – UNKNOWN EVIDENCE
The most important question asked about evidence found at a crime scene is: Where did this come from?
Known evidence consist of objects whose source or ownership is known at the time it is collected at the crime scene or elsewhere.
Unknown evidence refers to evidence discovered at a crime scene that has an unknown origin or source.
Unknown evidence must be compared to known evidence in order to validate the unknown’s origin.

14. INDIVIDUAL – CLASS EVIDENCE
Individual Evidence: Evidence that can be associated with only one particular person or object; evidence that can be linked to a unique, specific source with a very high degree of probability
Examples:
Matching ridge characteristics of fingerprints
Matching striations of two different bullets from the same gun
The irregular edges of a broken object (paper, glass, etc.) that fit together like a jigsaw puzzle (this type of evidence is not found very often)

15. INDIVIDUAL – CLASS EVIDENCE CONTINUED
Class Evidence: Evidence that cannot be associated with one particular object or person. It can only be put into a group of similar pieces of evidence.
Individual and class evidence is based on probability. The major weakness in class evidence is the fact that there is not enough statistical data (about the frequency of such evidence) to calculate or even estimate such probabilities.
The way to increase the probative value of class evidence is to find as many different types of class evidence as possible to link the suspect to the crime.

16. IDENTIFICATION
There are two types of analysis that forensic scientists do on evidence:
Identification
Comparison
Identification is a process of discovering chemical and/or physical properties of a piece of evidence. It is always performed on all evidence, known or unknown.
Example: Fingerprints, fabric, and bullets must all be identified and then compared to a known sample for individualism.

17. IDENTIFICATION CONTINUED
There are a few points that must be considered in cases where individualism is possible:
There is no set number of unique points that must be present for individualism.
With the exception of DNA, there is no statistical data for any of these evidence types than can support the certainty of a conclusion. For example, there is no data about the uniqueness of a particular fingerprint or shoeprint or bullet than would allow an examiner to state that the unknown matched the known to a degree of 90 or 95 or 98% certainty. Therefore, you cannot calculate the rate of error.

18. IDENTIFICATION CONTINUED
There has been insufficient rigorous scientific validation of the principles that underlie the conclusions of individuality of these evidence types (except for DNA). For example, in a recent case in Maryland, a judge ruled fingerprint evidence inadmissible because there is no scientific basis for the conclusion of individuality of fingerprints.

19. Types of Individual Evidence and Their Major Characteristics
Individualizable Evidence
Individualizing Characteristics
Fingerprints
Levels 2 and 3 minutiae details
Handwriting
Individual, unique style and characteristics
Shoe prints and tire treads
Unique characteristics that develop with time, damage, and wear
Large pieces of paint or glass or paper, etc.
Fracture or tear match with unique edge characteristics
DNA
Rarity of any DNA type makes probability of two people with same DNA very low

20. Types of Class Evidence and Their Major Characteristics
Why Not Individual?
Tiny glass or paint fragments
Too small to fracture match; no unique chemical or physical properties
Soils
No classification system; too much variability between nearby samples
Hairs and fibers
No unique characteristics; hair can be individualized if DNA is present
Illicit drugs, explosives, fire residues, etc.
Can identify all chemical components but no unique characteristics

21. DIRECT AND INDIRECT EVIDENCE
Direct Evidence – evidence that establishes a fact
Eyewitness testimony or victim’s testimony
Confessions
Physical evidence found on a person
Audio or visual recording of the act or crime

22. DIRECT AND INDIRECT EVIDENCE CONTINUED
- Circumstantial Evidence
Requires that a judge and/or jury make inferences about what transpired at the scene of a crime
Not definitive proof
Physical evidence is nearly always circumstantial, so evidence analyzed forensically is mostly circumstantial evidence
- Inference example: fingerprints or hairs found at the scene are consistent with that of a perpetrator; jurors may infer that the print or hair belongs to the defendant after this evidence is presented, linking the defendant to the scene

23. INFLUENCE OF DIRECT VS INDIRECT EVIDENCE
Circumstantial evidence is more objective, while direct evidence is subjective
In general, direct evidence is not considered to be as reliable as circumstantial evidence
Eyewitnesses can be deficient when identifying perpetrators or remembering certain events
Flawed questioning techniques can lead to erroneous testimony and confessions
The age of the eyewitness and the passing of time since the event can also lead to faulty testimony

24. INFLUENCE OF DIRECT VS INDIRECT EVIDENCE CONTINUED
Court research comparing the impact of the type of evidence differs it its results
Some research say direct (eyewitness) evidence has a stronger impact on the jury
At other times, physical evidence was considered more valid by jurors
Some studies suggest that having both types of evidence is no more compelling than having strong evidence of only one type
The direct evidence of a police officer’s testimony (concerning the source of evidence, documentation, and chain of custody) corroborates and strengthens the probative value of physical evidence

25. POSITIVE AND NEGATIVE CONTROLS
A false positive test is one that comes out positive when it should be negative. For example, the phenolphthalein test for the presence of blood can give false positive results.
How do you detect a false positive?
You do the same test on a negative control. A negative control is a material, usually a matrix, that would be expected to respond negatively to a particular chemical test.
In this case with the bloody shirt, you would perform the same phenolphthalein test on a part of the shirt that does not have blood on it.

26. POSITIVE AND NEGATIVE CONTROLS CONTINUED
A negative control should ALWAYS be run whenever a chemical test is being run on a substance that is mixed with a matrix such as the shirt. This guards against the matrix being the reason for the positive test.

27. POSITIVE AND NEGATIVE CONTROLS CONTINUED
Using the same example (the bloody shirt), what if the test comes out negative?
Either the stain is not blood or the test is not working correctly. In the second case, the failure of the test to react to what is actually blood would be called a false negative test.
A false negative test is a chemical test that turns out negative even though it should have been positive.
To make sure the test is working properly, the test is also performed on a positive control. A positive control is a material or chemical expected to respond positively to a particular chemical test.

28. POSITIVE AND NEGATIVE CONTROLS CONTINUED
Positive controls should always be run any time a chemical test is used to avoid false negative results.
Which incorrect test do you think is more serious?
False Positive or False Negative

29. UNIT SUMMARY
Evidence collected at a crime scene can be either known or unknown evidence.
All evidence undergoes identification analysis and is then put into successively smaller classes.
Once identified, some evidence can then be individualized.
Many analytical tests must be verified to make sure that they are working properly.