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

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.

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.

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.

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

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 - .26 x .85 x .02 = .0044 or 0.44%, or less than 1 out of 200 people would be expected to have this combination of blood factors

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.

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

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.

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?

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.

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

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.

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)

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.

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.

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.

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.

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

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

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

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

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

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

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.

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.

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.

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

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.