Evidence Characteristics Notes
Identification and Comparison
Identification is to determine identity and origination
Comparison is to determine similarity
Identification
The process of determining a substance’s physical or chemical identity beyond a
reasonable doubt. It must exclude all but one substance.
Testing procedures need to be adopted and regulated:
Examples: Drug analysis, Species determination, Explosive residue analysis
Comparison
The process of ascertaining whether two or more objects have common origin.
Evidence is compared to standard or reference samples
Ex. Hairs found at a crime scene are compared to those from a suspect
Paint chips on a victim’s garment compared to the pain of a suspect’s car.
Two Steps of Comparison:
1. Combinations of select properties are chosen from the suspect and reference.
2. A conclusion is based on the outcome of various tests of origin based on
probability
Probability
Probability has a large role in determining the origins of two or more specimens.
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.
Product Rule
Multiplying together the frequencies of independently occurring events to obtain an
overall frequency of occurrence for an event profile. Often determined from blood or
other biological materials.
Example: Two blood specimens are compared:
Both are found to be of human origin, both are typed as A.
Type A blood is found with a frequency of 26% in the population (not enough to
convict). However, we can look at other blood factors of the specimens to see if
they compare as well.
OJ Simpson Case: A blood stain found at the
crime scene showed the following
characteristics that compared to OJ’s blood, but
not the victims.
OJ Simpson Case: What is the probability of a
person having all of these factors?
(use the product rule) 0.26 x 0.85 x 0.02 = 0.0044 or .44%
One person in 200 (.44%) would be expected to have these characteristics.
Classification of Evidence
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.
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.
Example: 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.
Evidence that can be pinpointed to a specific source
Fingerprints, DNA, bullet markings
Class Characteristics
Evidence that can only be associated with a group and not a single source.
Fibers, hairs, blood type
Most evidence is of the class type. To be valuable, you need as many pieces of evidence
as possible. As the number of evidence objects increases, the probability of a person’s
involvement increases.
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?
There are very few statistical data available from which to derive this information, and
in a mass-produced world, gathering this kind of date is increasingly elusive.
Surprising 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.
One of the primary endeavors of forensic scientists must be to create and update
statistical 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 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. 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.
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? These are all questions that defy simple answers and are all the basis of
arguments.
Forensic Databases
The Integrated Automated Fingerprint Identification System (IAFIS)
The Combined DNA Index System (CODIS)
The National Integrated Ballistics Information Network (NIBIN)
The International Forensic Automotive Paint Data Query (PDQ)
SCIAR (shoeprint image capture and retrieval)
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.
Problems
Most items of physical evidence cannot be linked to a single person or object. Much of
the evidence is subjective in nature and subject to bias or human error.
Exclude or Exonerate
Physical evidence is given great weight in a court trial. Physical evidence can exclude or
exonerate a person from suspicion.