3 Physical Evidence

Physical Evidence Physical evidence encompasses any and all objects that can establish that a crime has or has not been committed or can provide a link between a crime and its victim or a crime and its perpetrator. Almost anything can be physical evidence. Responsibility of the investigator to recognize physical evidence and properly preserve it for laboratory examination, no amount of sophisticated laboratory instrumentation or technical expertise can salvage the situation damaged evidence.

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 as ultimately the weight will be decided by a jury. Collect Everything

Types of Physical Evidence Blood, semen, and saliva Documents Drugs Hair Impressions Organs and physiological fluids Paint Petroleum products Powder residues Soil and minerals Explosives Fibers Fingerprints Firearms and ammunition Glass Plastic bags Plastic, rubber, and other polymers Wood and other vegetative matter Tool marks Vehicle lights

Purpose of Examining Physical Evidence The examination of physical evidence by a forensic scientist is usually undertaken for identification 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.

Purpose of Examining Physical Evidence 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.

Identification 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.

Identification…. Second, identification requires that the number and type of tests needed to identify a substance be sufficient to exclude all other substances.

Common Types of Identification the chemical composition of an illicit drug. identify gasoline in residues recovered from the debris of a fire, identify the nature of explosive residues—for example, dynamite or TNT.

Common Types of Identification The identification of Blood Semen Hair Wood Also may ask to determine species of origin….where did it come from.

Comparison 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 examination 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 appreciate the role that probability has in ascertaining the origins of two or more specimens. Simply defined, probability is the frequency of occurrence of an event. With many analytical processes exact probability is impossible to define. In flipping a coin, probability is easy to establish. Not so with analytical processes.

Classifying Characteristics Individual Characteristics Evidence that can be associated to a common source with an extremely high degree of probability

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: The matching ridge characteristics of two fingerprints

The comparison of random striation markings on bullets or tool marks

Individual Characteristics Examples: The comparison of handwriting characteristics The fitting together of the irregular edges of broken objects in the manner of a jigsaw puzzle The comparison of irregular and random wear patterns in tire or footwear impressions

Matching sequentially made plastic bags by striation marks running across the bags

Class Characteristics Evidence associated only with a group is said to have class characteristics. Blood typing is a class characteristic, however if other blood factors are considered and used, the probability that the blood sample comes come from a common source increases using the product rule. Product rule – take the product of all the frequencies of a characteristic, this product is the probability of a common source. Used in OJ Simpson trial.

Class Evidence One of the current weaknesses 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.

Class Evidence For example, 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, and in a mass-produced world, gathering this kind of data is increasingly elusive.

Class Evidence The value of class physical 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 chances are low of encountering two indistinguishable items of physical evidence at a crime scene that actually originated from different sources.

Class Evidence 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. Finally, the contribution of physical evidence is ultimately determined in the courtroom.

From Class to Individual How much information (testing) is needed for evidence to go from class to individual classification (crossing over)?

Crossing Over 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?

Crossing Over These are all questions that defy simple answers and are the basis of arguments.

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.

Natural vs. Evidential Limits Limits to comparison… 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.

Using Physical Evidence As the number of different objects linking an individual to a crime scene increases, so does the likelihood of that individual’s involvement with the crime.

Using Physical Evidence 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.

Reconstruction Upon analysis of evidence it may be necessary to reconstruct the crime scene The method used to support a likely sequence of events by the observation and evaluation of physical evidence, as well as statements made by those involved with the incident, is referred to as reconstruction. Crime-scene reconstruction relies on the combined efforts of medical examiners, criminalists, and law enforcement personnel to recover physical evidence and to sort out the events surrounding the occurrence of a crime.