Chapter 3 - Nature of Evidence PDF

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Chapter 3 Notes from Crime and Forensics taught by Professor Forsthoff. ...

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Classification of Evidence ➢ The class or type of evidence can be very important in determining what value it has, how it should be collected, what else should be collected and most important, what conclusions can be drawn from a scientific examination of the evidence. ➢ These schemes apply to all evidence, not just scientific or chemical: ■ Physical-nonphysical ■ Real-demonstrative ■ Known-unknown ■ Individual-class ➢ Physical-Nonphysical Evidence ■ Physical evidence consists of objects or things. ■ Nonphysical evidence is verbal testimony about a crime or it may be someone’s actions during a crime. ■ 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. ■ Courtroom dramas often focus on motive: why someone would commit a crime? ■ A motive is often required as an element of crime. ➢ Real-Demonstrative Evidence ■ Real evidence is that which is generated by criminal activity. ■ It is found at the crime scene or elsewhere and pertains to the crime. ■ It may be fingerprints left at the scene or those obtained from a suspect. ■ It may be drugs or blood or bullets. ■ Real evidence, however, may not be found at the crime scene. ■ If someone is shot and killed and there is no weapon at the scene, a search of a suspect’s house may turn up the gun. ■ Demonstrative evidence is created to help explain or clarify real evidence. ■ It is produced after the crime and not by the crime. ■ Crime scene investigators always make sketches or videotapes of crime scenes and sometimes produce scale drawings or physical models if needed. ■ Demonstrations may be performed to illustrate the value or characteristics of evidence. ➢ Known-Unknown Evidence ■ The most important question that is asked about evidence found at a crime scene is: where did this come from? In other words: from what person or object did this arise? ■ The technical term for this source information is provenance.

The value of every piece of real evidence arises from its association with someone or something that was involved in the crime. ■ Crime scene reconstruction depends upon making these associations. ■ We use the term unkown evidence t o refer to evidence that is discovered at a crime scene and its origin or source is not known. ➢ Individual-Class Evidence ■ One way of defining individual evidence is that it is evidence that could have arisen from only one source. ■ Class evidence is evidence that could have any of several possible sources. ■ In most cases, the number of possible sources is unknown. ■ If a fingerprint examiner concludes that the print on the wine glass came from the suspect’s right index finger, she is really saying that the probability that the fingerprint came from someone else is negligible. ➢ Questions Raised about the Analysis of Pattern Evidence by the NAS Forensic Science Committee ■ Some of the oldest examples of the admission of scientific evidence into US courts have been fingerprints, firearms, and questioned documents. ■ For more than 120 years, fingerprint examiners have made statements in court that a fingerprint found on an object at the scene of a crime CAME from a particular finger on the suspect. ■ The curious, or perhaps, disturbing situation with these types of conclusions is that in none of these types of evidence has there ever been credible SCIENTIFIC proof of the underlying principles of these evidence analyses. ■ In order for a conclusion of individualization of a fingerprint left at a crime scene to a known print to be valid, it must be shown that all fingerprints are demonstrably and measurably unique, or that the markings left on bullets and/or cartridges fired from a rifled weapon are unique to that weapon, or that every person’s handwriting is demonstrably and measurably different from everyone else’s. ■ There are several problems with these positions. ■ First, the actual (ground) truth of the source of such evidence is not known (except perhaps by the perpetrator) and so the forensic examiner’s conclusion cannot be independently verified. ■ Second, the limited examples of research that looks into these issues are conducted on pristine fingerprints or bullets/cartridges or large samples of handwriting. ■ Third, there are no current standards for what actually constitutes a concordance between known and unknown evidence. ■ Fourth, there is no statistical evidence of how common ridge patterns, bullet/cartridge markings, or handwriting characteristics are in ■

casework, making expressions of error rates, confidence intervals or certainty of conclusions, virtually impossible. ➢ Identification  nd comparison. ■ There are two major types of analysis: identification a ■ The first, identification is always performed on all evidence, known or unknown. ■ Identification is a process of discovering chemical, biological, and/or physical properties of a piece of evidence. ■ In the case of the fingerprint on the wine glass, the examiner would visualize the print and remove it from the wine glass and preserve it. ■ She would then examine it carefully, noting its size and shape and the patterns that the ridges on the print form. ■ All physical evidence types can be classified as being class or individual evidence. ■ There is also a list of some evidence types that are not individualizable along with why they are not. ■ In those cases where individualization is possible, there are a few points that have to be considered and kept in mind: ● There is no set number of unique points that must be present in the known and unknown for any of these evidence types. ● With the exception of DNA, there is no statistical data for any of these evidence types that can support the certainty of a conclusion. ◆ There is no data about the uniqueness of a particular fingerprint or shoe print or bullet that would allow an examiner to state that the unknown matched the known to a degree of 90% or 95% or 98% certainty. ◆ Since examiners do not know the certainty of their conclusions, they leave no room for error and cannot explain what the error rate is. ● There has been insufficient, rigorous, scientific validation of the principles that underlie the conclusions of individuality of these evidence types. ◆ There have also been trials where the validity of conclusions of individuality has been challenged for handwriting and fingerprints. ◆ Some courts have ruled that handwriting evidence has not been proven to be unique. ■ All scientific evidence is identified, meaning that relevant physical, biological, and chemical characteristics are described and documented. ➢ DNA Typing Situation ■

The science of DNA analysis has developed to the point that it is usually considered to be individual evidence. ■ Virtually every forensic science laboratory in the world recognizes this. ■ The mode of comparison of known and unknown biological samples for DNA is different than that for other types of potentially individualizable evidence. ■ The difference with DNA evidence is that the characteristics that are being compared are not unique to that one person. ■ When DNA is typed, comparisons are made between the known and unknown DNA at more than a dozen locations. ■ The structure of the DNA is determined at each of these locations. ■ These parts of human DNA are not unique but they are polymorphic; there are multiple forms within the human population. ■ Each person possesses one or two forms of DNA at each location and the status of each location is independent of the others. ■ We know that if a piece of known DNA and unknown DNA have the same forms at all 13 locations, then the probability that the two pieces of DNA have the same source is so astronomically high that all other possible sources are essentially eliminated. ■ What makes DNA different from other types of evidence is that there is quantitative data that helps in making associations of evidence with a particular source. Positive and Negative Controls ➢ A negative control is a known substance or material that would be expected to yield a negative result to a particular test. ➢ 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 a positive test. ➢ A positive control is a substance that would be expected to respond positively to the test. ➢ Positive controls should always be run any time a chemical test is used to avoid false negative results.  ■...