Ch. 3 Fundamentals of Forensic Science - The Nature of Evidence

The Nature of Evidence

Anything can be submitted for scientific analysis. In investigations, these items become samples and yield data that forensic scientists use for interpretation. Evidence samples and data, analyzed in forensic labs, follow different rules than those in non-forensic scientific labs.

Key Concepts:
- Context is central to how evidence is analyzed and interpreted in reconstructing criminal events.
- The scientific method is still applied by forensic scientists, as it is by other scientists.
Keywords: Class, Coincidental Associations, Common Source, Comparison, Contamination, Demonstrative Evidence, Direct Transfer, Evidence, False Negative (Type II error), False Positive (Type I error), Hypotheses, Identification, Indirect Transfer, Individualization, Known Evidence, Locard Exchange Principle, Negative Control, Persistence, Positive Control, Probative Value, Proxy Data, Questioned Evidence, Repeatability, Scientific Method, Testability, Trier of Fact

Introduction

Evidence is critical to a trial; it provides the foundation for the arguments the attorneys plan to offer.
It is viewed as the impartial, objective information that leads a judge or jury to their conclusions.

What Is Evidence?

Whoever determines guilt or innocence is called the trier-of-fact.
The trier-of-fact must decide whether or not the statements made by witnesses are true.
Evidence can be defined as information, whether in the form of personal testimony, the language of documents, or the production of material objects, that is given in a legal investigation to make a fact or proposition more or less likely.

Kinds of Evidence

Most evidence is real; generated as a part of the crime and recovered at the scene or at a place where the suspect or victim had been.
Examples: Hairs, fingerprints, paint, blood, and shoeprints
Demonstrative Evidence: Items of evidence created to augment or explain real evidence; help the trier-of-fact understand complex testimony.
- Examples: Diagrams of hair characteristics, a computer simulation of a crime scene, or a demonstration of bloodstain pattern mechanics

Other Varieties of Evidence:

Circumstantial evidence: Evidence based on inference and not on personal knowledge or observation.
Conclusive evidence: Evidence so strong as to overbear any other evidence to the contrary.
Conflicting evidence: Irreconcilable evidence that comes from different sources.
Corroborating evidence: Evidence that differs from but strengthens or confirms other evidence.
Derivative evidence: Evidence that is discovered as a result of illegally obtained evidence and is therefore inadmissible because of the primary taint.
Exculpatory evidence: Evidence tending to establish a criminal defendant’s innocence.
Foundational evidence: Evidence that determines the admissibility of other evidence.
Hearsay: Testimony that is given by a witness who relates not what he or she knows personally, but what others have said, and that is therefore dependent on the credibility of someone other than the witness.
Incriminating evidence: Evidence tending to establish guilt or from which a fact trier can infer guilt.
Presumptive evidence: Evidence deemed true and sufficient unless discredited by other evidence.
Prima facie evidence: (prī-m# fay-sh#) Evidence that will establish a fact or sustain a judgment unless contradictory evidence is produced.
Probative evidence: Evidence that tends to prove or disprove a point in issue.
Rebuttal evidence: Evidence offered to disprove or contradict the evidence presented by an opposing party.
Tainted evidence: Evidence that is inadmissible because it was directly or indirectly obtained by illegal means.

Levels of Evidence

Not all evidence is created equal; some items of evidence have more importance than others.
The context of the crime and the type, amount, and quality of the evidence will dictate what can be determined and interpreted.
Most of the items in our daily lives are produced or manufactured en masse, including biological materials (you have thousands of hairs on your body, for example).
This has implications for what can be said about the relationships between people, places, and things surrounding a crime.

Forensic Science Is History

Forensic science is a historical science: The events in question have already occurred and are in the past.
Forensic scientists do not view the crime as it occurs (unless they’re witnesses); they assist the investigation through the analysis of the physical remains of the criminal activity.
Many sciences, such as geology, astronomy, archaeology, paleontology, and evolutionary biology, work in the same way: No data are seen as they are created, but only the remnants, or proxy data, of those events are left behind.
Forensic scientists analyze evidence of past criminal events to interpret the actions of the perpetrator(s) and victim(s).
Crime scene evidence is like a pronoun, grammatically standing in for a noun; evidence at a crime scene “stands in for” the actual items or are indicative of the actions taken at the scene.
The nature and circumstances of the crime will guide the crime scene investigators and the forensic scientists to choose the most relevant evidence and examinations.

Forensic Science vs. Other Applications of Science:

Forensic science is the demonstration of relationships between people, places, and things involved in legal cases through the identification, analysis and, if possible, individualization of evidence.
Historical nature, proxy data usage, or shared techniques with other sciences do not automatically qualify something as forensic science.

The Basis of Evidence: Transfer and Persistence

When two things come into contact, information is exchanged.
This exchange of information occurs, even if the results are not identified or are too small to have been found (Locard, 1930).
Developed by Edmund Locard, a French forensic microscopist in the early part of the twentieth century
The results of such a transfer would be proxy data: not the transfer itself, but the remnants of that transaction.
Essentially all evidence is transfer evidence.

Conditions Affecting Transfer Amounts:

The pressure applied during contact.
The number of contacts (six contacts between two objects should result in more transferred material than one contact)
How easily the item transfers material (mud transfers more readily than does concrete)
The form of the evidence (solid/particulate, liquid, or gas/aerosol)
How much of the item is involved in the contact (a square inch should transfer less than a square yard of the same material).
Direct Transfer: Evidence that is transferred from a source to a location with no intermediaries; transfers from A to B.
Indirect Transfer: Involves one or more intermediate objects; the evidence transfers from A to B to C. Indirect transfer can become complicated and poses potential limits on interpretation.
- It may be more accurate to speak of direct and indirect sources, referring to whether the originating source of the evidence is the transferring item
The second part of the transfer process is persistence.
Once the evidence transfers, it will remain, or persist, in that location until it further transfers (and, potentially, is lost), degrades until it is unusable or unrecognizable, or is collected as evidence.

How long evidence persists depends on the following:

What the evidence is (such as hairs, blood, toolmarks, accelerants)
The location of the evidence
The environment around the evidence
Time from transfer to collection
“Activity” of or around the evidence location
Numerous fiber transfer studies demonstrate that, from the time of transfer with normal activity, after about 4 h, $80\%$ of the transferred fibers are lost.
Transfer and persistence studies with other evidence types have shown similar loss rates

Contamination

Once the activity surrounding the crime has stopped, any transfers that take place may be considered contamination, that is, an undesired transfer of information between items of evidence.
Every contact produces some level of exchange, including contamination.
It is nearly impossible to completely prevent contamination, but it can be severely minimized through properly designed facilities, adequate protective clothing, and quality-centered protocols that specify the handling and packaging of evidence.

Identity, Class, and Individualization

All things are considered to be unique in space and time. No two (or more) objects are absolutely identical.
Identification: The examination of the chemical and physical properties of an object and using them to categorize the object as a member of a group.
- What is the object made of? What is its color, mass, and/or volume?
- All the characteristics used to identify an object helps to refine that object’s identity and its membership in various groups.
Class: A group of objects with similar characteristics.
Stating that two objects share a class identity may indicate they come from a common source.
- What is meant by a “common source” depends on the material in question, the mode of production, and the specificity of the examinations used to classify the object.
Individualization: When an object can be classified into a group with only one member (itself). An individualized object has been associated with one, and only one, source: It is unique.

The concept of individualization rests on two assumptions:

All things are unique in space and time; and
The properties by which a thing is classified are constant over time.

Known and Questioned Items

Questioned Evidence: We don’t know the original source of the paint.
Known Evidence: It is known where the sample originated.
It is their context that makes them questioned or known.

Relationships and Context

The relationships between the people, places, and things involved in crimes are critical to deciding what to examine and how to interpret the results.
It is important to establish the context of the crime and those involved early in the investigation.
This sets the stage for what evidence is significant, what methods may be most effective for collection or analysis, and what may be safely ignored.
Using context for direction prevents the indiscriminate collection of items that clog the workflow of the forensic science laboratory.
Stranger-on-stranger crimes beg the question of coincidental associations: two things which previously have never been in contact with each other have items on them which are analytically indistinguishable at a certain class level.

Comparison of Evidence

Identification: The process of discovering physical and chemical characteristics of evidence with an eye toward putting it into successively smaller classes.
Comparison: Done to try to establish the source of evidence. The questioned evidence is compared with objects whose source is known.
Goal: To determine whether or not sufficient common physical and/or chemical characteristics exist between the samples. If they do, it can be concluded that an association exists between the questioned and known evidence.

The strength of this association depends on a number of factors, including the following:

Kind of evidence
Intra- and inter-sample variation
Amount of evidence
Location of evidence
Transfer and cross-transfer
Number of different kinds of evidence associated to one or more sources
Individualization occurs when at least one unique characteristic is found to exist in both the known and the questioned samples.
Individualization cannot be accomplished by identification alone.

Controls

Controls are materials whose source is known and which are used for comparison with unknown evidence.
Controls are often used to determine if a chemical test is performing correctly.

Two types of controls:

Positive control: Is a material that is expected to give a positive result with the test reagents and serves to show that the test is working properly.
Negative control: For the shirt; it is expected that the results of the test would come out negative.
- Other negative controls can be run on “blank” samples, that is, those prepared similarly to the test materials being used but without any sample present.
If a negative control is not used, a false positive may be the result; that is, it may be concluded that the stain is blood when it is not. This gives rise to what statisticians call a Type I error.
Failure to run a positive control can cause a false negative result. This can give rise to what is called Type II error.

Analysis of Evidence: Some Preliminary Considerations

Science is a way of examining the world and learning about it.
The process of science, the scientific method, is proposing and refining of plausible explanations about any unknown situation. It involves asking and answering questions in a formal way and then drawing conclusions from the answers.

Science, through its method, has two hallmarks:

The first is the questions that are asked must be testable (or have testability).
The second hallmark of science is repeatability.
In the language of science, the particular questions to be tested are called hypotheses.
All relevant data will be collected without regard to whether it favors the hypothesis.
Once collected, the data will be carefully examined to determine what value they have in proving or disproving the hypothesis; this is the probative value of the data.
Evidence analysis proceeds by forming many hypotheses and perhaps rejecting some as the investigation progresses.

Some preliminary questions must be answered before we even begin to formulate hypotheses:

Is there sufficient material to analyze?
If extremely large amounts of material are submitted as evidence, how are they sampled?
What happens in cases in which more than one kind of analysis must be done on the same item of evidence?