Intro to Forensic Science Week 3.pdf

Physical Evidence Physical evidence encompasses any all objects that can estbaish that a crime has or has not been commiited or can provide a link between a cime and its victim or a crime and its perpetrator If the investigator cannot recognize physical evidnece or cannot properly preserve it for la...

Physical Evidence Physical evidence encompasses any all objects that can estbaish that a crime has or has not been commiited or can provide a link between a cime and its victim or a crime and its perpetrator If the investigator cannot recognize physical evidnece or cannot properly preserve it for laboratory exmaiation, no amount of sophisticated laboratory instrumentation or ehcincal expertise can salvage the situation It would be impossible to list all the objects that could conceivable be of importance to a crime Almsot anything can be physical evidence Although you cannout reloy on a list of categories it is useful to discuss some of the msot common types of physical evidence The purpose of recognzing physical evidence is so that it can be collected and analyzed It is difficult to ascertain the wright a given piece of evidence will have in a case as ultimately the weight will be decided by a jury Types of Physical Evidence Blood, semen, and slaiva Docuemnts Drugs Exploaives Fibers Fingerpritns Firearms and ammunition Glass Hair Impressions (bite marks, footprints, ear prints ) Organs and physiological fluids Paint Petroleum products Plastic bags Plastic, rubber and other polymers Powder residues Soil and minerals Tool marks Vehicle lights Wood and other vegetative matter Purpose of Examining Physical Evidence The examination of physical evidence by a forensic scientist is ually underetaken for idnetiidcaition or comparison purposes Identification has as its purpose 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 exminatons for the ultimate purpose of determining whether or not they have a common origin Identification The object of an identification is to determine the physical or chemical identity with as near absolute certainty as existing analytical ttechniques will permit -Th process of idnetificagtion first requires the adoption of testing procedures that give characteristic results for specific standard materials The object of an idetnifcation is to determine the physical or chemical identity with ass near absolute certainty as existing analytic techniques will permit -once these test results have been established they may be permanelty recorded and used repeatedly to prove the identity of suspect materials The object of an identification is to determine the physical or chemical identity with as near absolute certainty as editing analytical techniques will permit -Second, identification rewuires that the number and type of tests needed to identity a substance be sufficient to exclude all other substances Common Types of Identification The crime labortory is frequently requested to identify the chemical composition of an illicit drug It may be asked to identify gasoline in residues recovered from the debris of a fire or it may have to identify the nature of explosive residues-for example, dynamite or TNT The identification of blodd, semen, hair or wood are also very common and as a matter of routine include a determination for species origin Comparison A comparative analysis has the important role of determining whether or not a suspect specimen have a common origin Both the standard/reference and the suspect specimen are subject to the same tests The forensic comparison is actually a two step-procedure -First, combinations of select properties are chosen from the suspect and the standard/ reference specimen for comparison -Second, once the exainaiton has been completed the forensic scientist must be prepared to render a conclusion with respect to the origins Role of Probability To comprehend the evidential value of a comparison one must appreciaagt the role probability has in ascertaining the orins of two or more specimens Simply defined, 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 Characterisitics Individual Characteristics -Evidence that can be associated to a common source with an extremely high degree of probability is said to possess individual characteristics Individual Characteristics In all cases, it is not possible to state with mathematiccal exactness the probability that the specimens are of common origin It can only be concluded that this probability is so high as to defy mathmaticial calculations or human comprehension Examples: The matching ridge characteristics of two fingerprints The comparison of random straton markings on bullets or tool marks The comparison of irregular and random wear patterns in tire or footwear impressions The comparion of handwriting charceteristis The fitting together of the irregular edges fo broken objects in the manner of a jigsaw puzzle Matchhing sequentially made plastic bags by stration marks running across the bags Class Charcteristics Class Characteristics Evidence associated only with a group is said to have class characteristics Class Evidence One of the current weaknesses of forensic science is the inability off the examiner to assign exact or even approximate probabulty values to the comparison of most class physical evidence For example what is the probability that a nylon fiber oginatef from s particular sweater r paint chip came from a suspect car in a hit and run? There are very few statistical data available from which to derfive this information and in a mass-produced world, gathering this kind of data is increasingly elusive One of the primary endeavors of forensic sciengtists must be create and update statisial 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 evidence lies in its ability to provide corroboration of events with data that are, as nearly as possible free of human error and bias The chaces are low of encountering two indistingushibale items of physical evidence at a crime scne thagt actually orginated from different sources When one is dealing with more than one type of class evidence, their collective presence may lead to an extremely igh certainty that they originated from the same source Finally the contriubutoon of physical evidence is ultimately determined in the courtroom Crossing over Crossing over the line from class to individual does not end the discussions How many strations are necessary to individualize a mark to a single tool and no other? -How many colour layers indivulaize a paint chip to a single car? -How many ridge characteristics individulaize a fingerprint? -How many handwriting characteristics tie a person to a signature? These are all questions that defy simple answers and are the basis of arguments Natural vs. Evidential Limits There are practical limits to the propersities and charcteists 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 pof evidential use from natural variations is not always an easy task Using Physical Evidence As the number of different objects liking an individual to a crime scne increases, so does the iklihood of that indivuas involvement with the crime Just as important, 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 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 Newtwork (NIBIN) allows firearm analysts to acquire, digitize and comapre markings madde by a firearm on buullets and cartridge casings The International Forensic Automotive Paint Data Query (PDQ) databases conians chemical and colour information pertaining to original automotive paints SICAR ( Shoeprint image capture and retrieval) is a shoeprint database