Chapter 3: 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

No amount of sophisticated laboratory instrumentation or technical expertise can salvage the situation

What happens if the investigator cannot recognize physical evidence or cannot properly preserve it for laboratory examination?

Jury

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…

Identification

The object is to determine the physical or chemical identity with as near absolute certainty as existing analytical techniques will permit

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

What does the process of identification require first?

The test results may be permanently recorded and used repeatedly to prove the identity of suspect materials

What happens after the first step of the identification process?

The number and type of tests needed to identify a substance should be sufficient to exclude all other substances

What does the identification process require second?

The Crime Laboratory

Frequently requested to identify the chemical composition of an illicit drug

Identification of Dynamite or TNT

The crime laboratory 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

Blood, semen, hair, or wood

What types of physical evidence are commonly identified?

Comparative Analysis

Has the important role of determining whether or not a suspect specimen and a standard/reference specimen have a common origin

Subjected to the same tests

Both the standard/reference and the suspect specimen are…

The Forensic Comparison

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

Appreciate the role that probability has in ascertaining the origins of two or more specimens

What must one do to comprehend the evidential value of a comparison?

Probability

The frequency of occurrence of an event

Impossible to define

With many analytical processes, exact probability is…

Individual Characteristics

Evidence that can be associated to a common source with an extremely high degree of probability is said to possess…

It can only be concluded that this probability is so high as to defy mathematical calculations or human comprehension

Examples: The matching ridge characteristics of two fingerprints

In all cases, it is not possible to state with mathematical exactness the probability that the specimens are of common origin, so…

Class Characteristics

Evidence associated only with a group is said to have

The inability of the examiner to assign exact or even approximate probability values to the comparison of most class physical evidence

What is one of the current weaknesses of forensic science?

To create and update statistical databases for evaluating the significance of class physical evidence

What is one of the primary endeavors of forensic scientists?

Be linked definitively to a single person or object

Most items of physical evidence retrieved at crime scenes cannot…

Provide corroboration of events with data that are, as nearly as possible, free of human error and bias

The value of class physical evidence lies in its ability to…

Their collective presence may lead to an extremely high certainty that they originated from the same source

What is typically the case when one is dealing with more than one type of class evidence?

The Courtroom

The significance of physical evidence is ultimately determined in…

The source of heated debate and honest disagreement among forensic scientists

Just when an item of physical evidence crosses the line that distinguishes class from individual is a difficult question to answer and is often…

Different Sources

The chances are low of encountering two indistinguishable items of physical evidence at a crime scene that actually originated from…

Properties and characteristics the forensic scientist can select for comparison

There are practical limits to the…

Natural variations in objects become almost infinite

Modern analytical techniques have become so sophisticated and sensitive that…

No two things in this world are alike in every detail

When carrying natural variations to the extreme…

Natural Variations

Evidential variations are not the same as

So does the likelihood of that individual’s involvement with the crime

What happens as the number of different objects linking an individual to

a crime scene increases?

Physical evidence collected at a crime scene is found to be different from standard/reference samples collected from that subject

A person may be exonerated or excluded from suspicion if…

Integrated Automated Fingerprint Identification System (IAFIS)

A national fingerprint and criminal history system maintained by the FBI

Combined DNA Index System (CODIS)

Enables federal, state, and local crime laboratories to electronically exchange and compare DNA profiles

Genealogy Databases (GEDmatch)

Are of emerging use to identify close relatives using DNA profiles; These databases contain samples that are processed by commercial genealogy companies and uploaded by private citizens

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)

A shoeprint database

Identification

The process of _________ determines a substance’s physical or chemical identity with as near absolute certainty as existing analytical techniques will permit

Eliminate

The number and type of tests needed to identify a substance must be sufficient to _________ all other substances from consideration

Comparison

A(n) _________ analysis subjects a suspect specimen and a standard/reference specimen to the same tests and examination in order to determine whether they have a common origin

Probability

_________ is the frequency of occurrence of an event

Individual

Evidence that can be traced to a common source with an extremely high degree of probability is said to possess _________ characteristics

Class

Evidence associated with a group and not with a single source is said to possess _________ characteristics

True

One of the major deficiencies of forensic science is the inability of the examiner to assign exact or approximate probability values to the comparison of most class physical evidence

Reconstruct

The value of class physical evidence lies in its ability to _________ events with data in a manner that is, as nearly as possible, free of human error and bias

Weight

The _________ accorded physical evidence during a trial is left entirely to the trier of fact

Experience and judgment

Although databases are consistently updated so that scientists can assign probabilities to class evidence, for the most part, forensic scientists must rely on _________ when interpreting the significance of class physical evidence

Eyewitness

The believability of _________ accounts, confessions, and informant testimony can all be disputed, maligned, and subjected to severe attack and skepticism in the courtroom.

False

Physical evidence cannot be used to exclude or exonerate a person from suspicion of committing a crime

False

The distinction between individual and class evidence is always easy to make

True

Given the potential weight of scientific evidence in a trial setting, failure to take proper safeguards may unfairly prejudice a case against the suspect

Hands-on experience

Students studying forensic science must be cautioned that merely reading relevant textbooks and journals is no substitute for _________ in this most practical of sciences

Variations

Modern analytical techniques have become so sensitive that the forensic examiner must be aware of the _________ among materials when interpreting the significance of comparative data

False

A fingerprint can be positively identified through the IAFIS database

CODIS

A database applicable to DNA profiling is _________

NIBIN

The _________ database allows firearm analysts to compare markings made by firearms on bullets that have been recovered from crime scenes

PDQ (Paint Data Query)

The _________ database contains chemical and color information pertaining to original automotive paints

a. Screening tests can quickly indicate the presence of an illicit drug

c. Identifying heroin requires more specific confirmatory tests

b. Determining full composition requires comprehensive chemical analysis

Arrange the following tasks in order from the one that would require the least extensive testing procedure to the one that would require the most extensive. Explain your answer.

a. Determining whether an unknown substance contains an illicit drug

b. Determining the composition of an unknown substance

c. Determining whether an unknown substance contains heroin

a. 0.036 × 0.081 × 0.089 ≈ 0.000259

b. 0.062 × 0.039 × 0.085 ≈ 0.000205

c. 0.112 × 0.028 × 0.067 ≈ 0.000210

The following are three possible combinations of DNA characteristics that may be found in an individual’s genetic profile. Using the product rule, rank each of these combinations of DNA characteristics from most common to least common. The number after each characteristic indicates its percentage distribution in the population.

a. New automobile tire impression → Class

b. Fingerprint → Individual

c. Spent bullet cartridge → Individual

d. Mass-produced synthetic fiber → Class

e. Pieces of a shredded document → Individual

f. Commercial potting soil → Class

g. Skin and hair scrapings → Individual

h. Fragments of multilayer custom automobile paint → Individual

For each of the following pieces of evidence, indicate whether the item is more likely to possess class or individual characteristics and explain your answers.

a. An impression from a new automobile tire

b. A fingerprint

c. A spent bullet cartridge

d. A mass-produced synthetic fiber

e. Pieces of a shredded document

f. Commercial potting soil

g. Skin and hair scrapings

h. Fragments of a multilayer custom automobile paint

Class-characteristic databases: PDQ (paint), Fiber and chemical databases

Individual-characteristic databases: CODIS (DNA), IAFIS (fingerprints), NIBIN (firearms)

Which of the forensic databases described in the text contain information that relates primarily to evidence exhibiting class characteristics? Which ones contain information that relates primarily to evidence exhibiting individual characteristics? Explain your answers.

Blood samples — to compare victim and suspect DNA

Hair samples — to determine transfer during struggle

Fibers — to link suspect and victim clothing

Skin cells — for DNA comparison

An investigator at a murder scene notes signs of a prolonged struggle between the attacker and victim. Name at least three types of physical evidence for which the investigator would probably collect standard/reference samples and explain why he or she would collect them.