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Legal vs. Scientific Questions PHYSICAL EVIDENCE Questions Science May Answer What crime was committed? How was the crime committed? “Who dunnit? dunnit?” Where is the true “scene of the crime” crime”? When was the crime committed in time? The law must establish that a crime has been committed by defining the corpus delecti (body of the crime) All elements that legally define any particular crime must be present to proceed The forensic scientist must translate the legal question into a scientific question Question Science Can’ Can’t Answer Why was the crime committed? The Framework Determine the legally relevant question Determine the assumptions Analyze the evidence to provide facts about the question Make an inference based on the assumptions & facts Fundamental Principles Transfer Locard Exchange Principle Identification Individualization placing objects in a class narrowing the class to one 1 Probative Value of Evidence Fundamental Concepts of Forensic Science Association Reconstruction linking a person with a crime scene understanding the sequence in which events occurred Strength vs. Significance Strength of an association depends only on a pinhole view of the evidence can determine an exclusive relationship between a finger and an object on which a fingerprint is found Principle of Divisible Matter Significance of an association uses a widewide-angle lens & includes the circumstances of the crime a fingerprint on the outside of a stolen car is meaningless a fingerprint on the steering wheel of the stolen car is highly probative Creates physical traits not present in the original undivided material These new traits may be used to associate the two separated items at a later time When force is applied, matter divides When matter is divided into smaller units, it will Process of Division Crucial to articulate the significance of analytical similarities This is probably the most frequent contention between experts in the courtroom retain characteristics of the larger piece acquire characteristics created by the process of division Process of Division New traits created at the fracture boundary Boundary roughness is a natural consequence Boundary surfaces similar not completely complementary 2 Principle of Divisible Matter Corollary 1 Some characteristics retained by smaller pieces are unique to the original item or to the division process Principles of Divisible Matter Corollary 3 useful for individualizing all pieces to the original Corollary 2 Some characteristics retained by the smaller pieces are common to the original item as well as to other items of similar manufacture (class characteristics) Every time an object comes into contact with another object, it either leaves a portion of itself or takes a portion of the other object with it confounds the attempt to infer a common source useful for associating pieces to each other The Locard Exchange Principle Some characteristics from the original item will be lost or changed during or after the moment of division and subsequent dispersal Transfer Physical movement of material from one place to another 1 mm fiber transferred from a sweater (its source) to a sweatshirt (its destination) during contact Every contact leaves its trace Transfer No physical movement of material a pattern formed when an object makes an impression on some receptive substrate shoe stepping in mud (no detectable amount of leather left by shoe The principles of divisible matter and transfer are the two fundamental scientific principles upon which the forensic analysis of physical evidence are based Division & transfer of matter occurs before and during the crime 3 Mechanism of Transfer Example Mechanism of Transfer Example 9 a.m. -- hair falls from my head to floor 2 p.m. -- stranger walks across floor & hair picked up on shoe 7 p.m. -- hair is removed from shoe in an investigation Future time -- comparison of hair on shoe & reference hair from my head shows them to be indistinguishable Might infer that I and the person wearing the shoe were in contact A WRONG INFERENCE Mechanism of Transfer Example Scenario PrePre-pubescent girl asserts that her uncle that lives in the same house molested her. After event, she took a shower. The towel she used is collected as evidence. Pubic hair is present on the towel. Comparison of the evidence hair and reference hairs from the uncle show them to be indistinguishable. Mechanism of Transfer Example What is the significance of this comparison? How strong is the association? Mechanism of Transfer Example Mechanism of Transfer Example Method of hair examination used microscopic comparison DNA comparison Since the uncle lives in the house & uses that bathroom Inference of common source is strong Significant likelihood of finding his pubic hair on a towel in that bathroom Association is strong Significance of the association is minimal 4 Physical Evidence Types of Physical Evidence Almost anything can be physical evidence. The purpose of recognizing physical evidence is so that it can be collected and analyzed. It is difficult to ascertain the weight a given piece of evidence will have in a case Ultimately the weight will be decided by a jury. Blood, semen, and saliva Documents Drugs Explosives Fibers Fingerprints Firearms and ammunition Glass Hair Impressions Organs and physiological fluids The examination of physical evidence by a forensic scientist is usually undertaken for identification or comparison purposes. Identification : the determination of the physical or chemical identity of a substance with as near absolute certainty as existing analytical techniques will permit. 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 chemical composition of an illicit drug. Identify gasoline in residues recovered from the debris of a fire Identify the nature of explosive residues— residues—for example, dynamite or TNT. Identification of blood, semen, hair, or wood The object of an identification is to determine the physical or chemical identity with as much certainty as existing analytical techniques will permit. 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. Second, identification requires that the number and type of tests needed to identify a substance be sufficient to exclude all other substances. Comparison Common Types of Identification Paint Petroleum products Plastic bags Plastic, rubber, and other polymers Powder residues Soil and minerals Tool marks Vehicle lights Wood and other vegetative matter Identification Purpose of Examining Physical Evidence A comparative analysis has the role of determining whether or not a suspect specimen and a standard/reference specimen have a common origin. origin. Both the standard/reference and the suspect specimen are subject to the same tests. The forensic comparison is actually a twotwo-step procedure. First, combinations of select properties are chosen from the suspect and the standard/reference specimen for comparison. Second, once the examination has been completed, the forensic scientist must be prepared to render a conclusion with respect to to the origins. 5 Role of Probability To comprehend the evidential value of a comparison, you must appreciate the role that probability has in determining the origins of two or more specimens. Probability is the frequency of occurrence of an event. In flipping a coin, probability is easy to establish. With many analytical processes exact probability is impossible to define. Classifying Characteristics Individual Characteristics Evidence that can be associated to a common source with an extremely high degree of probability is said to possess individual characteristics. Class Characteristics Evidence associated only with a group is said to have class characteristics. characteristics. Individual Characteristics In all cases, it is not possible to state with mathematical exactness the probability that the specimens are of common origin. It can only be concluded that this probability is so high as to defy mathematical calculations or human comprehension. Individual Characteristics Examples: Class Evidence Class Characteristics Surprising is the frequent inability of the laboratory to relate physical evidence to a common origin with a high degree of certainty. Evidence is said to possess class characteristics when it can be associated only with a group and never with a single source. The high diversity of class evidence in our environment makes their comparison very significant in the context of a criminal investigation. The 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 Matching sequentially made plastic bags by striation marks running across the bags Weakness of forensic science is the inability of the examiner to assign exact or even approximate probability values to the comparison of most class physical evidence. What is the probability that a nylon fiber originated from a particular sweater, or that a paint chip came from a suspect car in a hit and run? There are very few statistical data available from which to derive this information 6 Class Evidence Forensic scientists must create and update statistical databases for evaluating the significance of class physical evidence. Most items of physical evidence retrieved at crime scenes cannot be linked definitively to a single person or object. The value of class physical evidence is its ability to provide corroboration of events with data that are free of human error and bias. Forensic Databases • The Integrated Automated Fingerprint Identification System (IAFIS), a national fingerprint and criminal history system maintained by the FBI. • The Combined DNA Index System (CODIS) enables federal, state, and local crime laboratories to electronically exchange and compare DNA profiles. • The National Integrated Ballistics Information Network (NIBIN) allows firearm analysts to acquire, digitize, and compare markings made by a firearm on bullets and cartridge casings. • The International Forensic Automotive Paint Data Query (PDQ) database contains chemical and color information pertaining to original automotive paints. • SICAR (shoeprint image capture and retrieval) is a shoeprint database. Class Evidence The chances are low of encountering two indistinguishable items of physical evidence at a crime scene that actually originated from different sources. When one is dealing with more than one type of class evidence, their collective presence may lead to an extremely high certainty that they originated from the same source. The contribution of physical evidence is ultimately determined in the courtroom. Using Physical Evidence Class Characteristic Example Cocaine in a container can be identified as cocaine by its chemical properties These properties are shared by all samples of the same molecular structure As the number of different objects linking an individual to a crime scene increases, so does the likelihood of that individual’ individual’s involvement with the crime. A person may be exonerated or excluded from suspicion if physical evidence collected at a crime scene is found to be different from standard/reference samples collected from that subject. Class Characteristic Example Cocaine from Columbia looks just like cocaine from any other source Sometimes (particularly in drug cases) identification of the evidence item is all that is required The source of the cocaine is not important its possession is illegal 7 Another Example White cotton fibers are so common that they will be present in almost all trace evidence collection Fiber can be identified as cotton via polarizing light microscope Cotton Example Origin from a specific source can’ can’t be determined without looking at some nonnoncotton characteristic such as trace elements Discovery of fibers that fall into the class of white cotton not very useful Last example A bullet can be identified as .22 caliber by measurement May be isolated to a specific manufacturer by its markings Bullet Example Manufacture usually produces millions of rounds of such ammunition can’ can’t distinguish one round from another by looking only at caliber Firearms examiner would look at individualizing characteristic such as bullet striae to increase the probative value Crossing Over Crossing over the line from class to individual does not end the discussions. How many striations are necessary to individualize a mark to a single tool and no other? How many color layers individualize a paint chip to a single car? How many ridge characteristics individualize a fingerprint? How many handwriting characteristics tie a person to a signature? Limitations The strength of the evidence is an inverse function of its limitations These are all questions that defy simple answers and are the basis of legal arguments. 8 Questions Setting Limitations What traits are being examined? What distinguishes this item from others in the same class? How many different kinds of traits exist & which of these have been examined? How much variability might exist within a single individual? How much variability exists in the population? The Role of Forensic Science Limitations The more common the observed traits, the less significant the similarity becomes Insensitivity of detection method Deteriorated state of sample inability to observe a difference that exists obliteration of a difference that actually existed Limitations Limitations The amount of information that can be derived about the sources of the carpet is limited by the type of examination performed the nature of the evidence Provide scientific information to the legal community Done via reports and testimony A statement of the limitations on conclusions drawn from an experimental result is an essential element Analyst examines the chemical & physical traits of an evidence fiber These traits are similar to fibers from a reference carpet in the suspect’ suspect’s house Analyst knows that thousands of yards of that carpet were installed in homes The evidence fiber is consistent with the reference carpet and the other thousands of carpets Natural vs. Evidential Limits There are practical limits to the properties and characteristics the forensic scientist can select for comparison. Modern analytical techniques have become so sophisticated and sensitive that natural variations in objects become almost infinite. Carrying natural variations to the extreme, no two things in this world are alike in every detail. Evidential variations are not the same as natural variations. Distinguishing variations of evidential use from natural variations is not always an easy task. 9 Responsibility of the Analyst May conclude that an evidence object & a reference object show similar traits Must understand and communicate the limitations inherent in the conclusion limitations are a combined function of the nature of the traits the sensitivity and resolution of the detection methods the state of the evidence 10