Fs Ch 5

Physical Evidence

Chapter 5

Forensic ScienceRichard Saferstein

© 2009 Pearson Education, Upper Saddle River, NJ 07458.

All Rights Reserved.2

Physical Evidence

Almost anything can be physical evidence

Important to recognize physical evidence is so that it can be collected and analyze

Difficult to ascertain weight of piece of evidence in court

-ultimately decided by a jury




Purpose of Examining Physical Evidence

Identification:-determination of the physical/chemical

identity of a substance with as near absolute certainty as existing analytical techniques will permit

Comparison: -subjects a suspect specimen and a

standard/reference specimen to the same tests and examinations to determine if they have a common origin




IdentificationPROCESS:

-adoption of testing procedures that give characteristic results for specific standard materials

-test results have been established-permanently recorded-used repeatedly to prove the identity of suspect

materials.-requires number/type of tests needed to identify

a substance exclude all other substances




Common Types of Identification Illicit drugs

Gasoline in residues

Explosive residues

Identification of blood/semen/hair




Comparison Comparative analysis:-if suspect specimen and a standard/reference

specimen have a common origin-standard/reference and suspect specimen subjected

to same tests

2 step procedure:-combinations of select properties are chosen from suspect and

standard/reference specimen for comparison-be prepared to render a conclusion with respect to the origins




Role of Probability

To comprehend the evidential value:-must appreciate the role that probability has

in ascertaining the origins of two or more specimens

Probability:-frequency of occurrence of an event Exact probability is impossible to define in

many analytical processes




Classifying Characteristics

Individual Characteristics-Evidence associated to a unique

source with high degree of probability

Class Characteristics-Evidence associated only with a

common source




Individual Characteristics

Not possible to state with mathematical exactness the probability that the specimens are of common origin

-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

random striation markings on bullets/tool marks Irregular/random wear patterns in tire/footwear

impressions handwriting characteristics sequentially made plastic bags by striation

marks running across the bags




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. Nevertheless, the high diversity of class

evidence in our environment makes their comparison very significant in the context of a criminal investigation.




Class Evidence Weaknesses of forensic science:-inability to assign exact/approximate

probability values to the comparison of most class physical evidence

Very few statistical data-available from which to derive this

information-mass-produced world




Class Evidence Primary endeavor:-create and update statistical databases

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.




Class Evidence Chances are low of encountering two

indistinguishable items of physical evidence at a crime scene that actually originated from different sources

More than one type of class evidence may lead to high certainty that they originated from the same source.

Contribution of physical evidence is ultimately determined in the courtroom.




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?

These are all questions that defy simple answers and are the basis of defense lawyer arguments




Natural vs. Evidential Limits

Practical limits to properties/characteristics for comparison:

-Modern analytical techniques are so sophisticated/sensitive that natural variations in objects become almost infinite

-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

Number of different objects linking an individual to a crime scene increases:

-likelihood of that individual’s involvement with the crime

-exonerated/excluded from suspicion if physical evidence is different from standard/reference samples collected from that subject




Forensic Databases• Automated Fingerprint Identification System (IAFIS):-fingerprint and criminal history system (FBI)

• Combined DNA Index System (CODIS):-electronically exchange and compare DNA profiles

• National Integrated Ballistics Information Network (NIBIN):-acquire/digitize/compare markings made by a firearm on bullets and

cartridge casings

• International Forensic Automotive Paint Data Query (PDQ):-chemical/color information of original automotive paints • SICAR (shoeprint image capture and retrieval)• -shoeprint database




Forensic Pathology Forensic Pathology:

-investigation of unnatural/unexplained/violent deaths

-Cause and manner of death-autopsy -toxicology




Forensic Pathology After a human body expires there are several

stages of death. Rigor mortis results in the shortening of muscle tissue

and the stiffening of body parts in the position at death (occurs within the first 24 hrs. and disappears within 36 hrs.).

Livor mortis results in the settling of blood in areas of the body closest to the ground (begins immediately on death and continues up to 12 hrs.).

Algor mortis results in the loss of heat by a body (a general rule, beginning about an hour after death, the body loses heat by 1 to 1 1/2 degrees Fahrenheit per hour until the body reaches the environmental temperature).




Beyond the Crime Scene

Beyond the crime scene to the autopsy room:

-medical examiner/coroner

-tissues/organs for pathological/toxicological examination

-secure items from the body for laboratory examination




Beyond The Crime Scene

The following are to be collected and sent to the forensic laboratory:

1. Clothing2. Fingernail scrapings3. Head/pubic hairs4. Blood (DNA/toxicology) 5. Vaginal/anal/oral swabs (sex-related

crimes)6. Bullets from the body7. Hand swabs (Gunshot Residue)




Forensic Anthropology Identification/examination of human skeletal

remains:-sex/approximate age/race/skeletal injury-create facial reconstructions to identify skeletal

remains

Sites of human remains treated as a crime scene-secured and thoroughly searched-locate all bones if scattered-footwear impressions-discarded items- aerial photography/metal detectors/ground-

penetrating radar/infrared photography/cadaver dogs




Forensic Entomology Study of insects:

-estimate the time of death-postmortem interval (PMI)-different insect species used to estimate PMI-all entomological/insect collected -temperature readings-collect specimens from every area in which they

are found

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