1. Chapter 3
PHYSICAL EVIDENCE

2. Physical Evidence
It would be impossible to list all the objects that could conceivably be of importance to a crime.
Almost anything can be Physical Evidence.
Although you cannot rely on a list of categories, it is useful to discuss some of the most common types of physical evidence.

3. 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.

4. Types of Physical Evidence
Blood, semen, and saliva
Documents
Drugs
Explosives
Fibers
Fingerprints
Firearms and ammunition
Glass
Hair
Impressions
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

5. 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.

6. 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.

7. Identification
The object of an identification is to determine the physical or chemical identity with as near absolute 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.

8. Identification
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.

9. Common Types of Identification
The crime laboratory 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.

10. Common Types of Identification
The identification of blood, semen, hair, or wood are also very common and, as a matter of routine, include a determination for species origin.

11. Comparison
A comparative analysis has the important role of determining whether or not a suspect specimen and a standard/reference specimen have a common origin.
Both the standard/reference and the suspect specimen are subject to the same tests.

12. 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.

13. 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.
In flipping a coin, probability is easy to establish.

14. Role of Probability
With many analytical processes exact probability is impossible to define.

15. 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.

16. 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.

17. Individual Characteristics
Examples:
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

18. Class Characteristics
Surprising to the inexperienced forensic scientist 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.
Here again, probability is a determining factor.

19. Class Characteristics
Nevertheless, the high diversity of class evidence in our environment makes their comparison very significant in the context of a criminal investigation.

20. 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.
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?

21. Class Evidence
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.
One of the primary endeavors of forensic scientists must be to create and update statistical databases for evaluating the significance of class physical evidence.

22. Class 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 lies in its ability to provide corroboration of events with data that are, as nearly as possible, free of human error and bias.

23. 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.
Finally, the contribution of physical evidence is ultimately determined in the courtroom.

24. 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?

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

26. 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.

27. Natural vs. Evidential Limits
Evidential variations are not the same as natural variations.
Distinguishing variations of evidential use from natural variations is not always an easy task.

28. 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.
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.

29. Forensic Databases
The Integrated Automated Fingerprint Identification System (IAFIS), a national fingerprint and criminal history system maintained by the FBI.
TheCombined 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.

30. Forensic Databases
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.

31. Reconstruction
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.

32. 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.

33. The Role of Physical Evidence
The physical evidence left behind at a crime scene plays a crucial role in reconstructing the events that took place surrounding the crime.
Although the evidence alone does not describe everything that happened, it can support or contradict accounts given by witnesses and/or suspects.

34. The Role of Physical Evidence
Information obtained from physical evidence can also generate leads and confirm the reconstruction of a crime to a jury.
The collection and documentation of physical evidence is the foundation of a reconstruction.

35. Summing It Up
Reconstruction is a team effort that involves putting together many different pieces of a puzzle.
The right connections have to be made among all the parts involved so as to portray the relationship among the victim, the suspect, and the crime scene.
If successful, reconstruction can play a vital role in aiding a jury to arrive at an appropriate verdict.

36. Summing It Up
The recognition, collection, and analysis of physical evidence is the foundation to successful reconstruction, but only part of the process.