Types of Evidence

““You can observe a lot just by watching”You can observe a lot just by watching”

- Yogi Berra

Terminology

Evidence – something that tends to establish or disprove a fact.

Testimonial Evidence – what is said in court by a competent witness; also called direct evidence

Physical Evidence – tangible items that tend to prove some material fact; also called real evidence

Indirect Evidence – evidence providing only a basis for inference about the disputed fact

Circumstantial evidence – evidence based on suggestion rather than personal knowledge or observation

Terminology – cont....

Question or unknown sample – material that has been collected from a known location but is of unknown origin

Known or control sample – material that comes from a proven or known source

Individual Evidence- - material that can be related to a single source

Class Evidence - -material that can be associated only with a group of items that share properties or characteristics

Probative Value – the ability of evidence to prove something that is material to the crime

I. The Locard Principle

A fundamental principle of investigation for every crime scene comes from Edmond Locard, (early 1900’s). believed that a criminal could be connected to a crime

by trace evidence collected at the crime scene “Whenever two objects come into contact, there is

always transfer of material. The methods of detection may not always be sensitive enough to demonstrate this , or the decay rate may be so rapid that all evidence of transfer has vanished after a given time. Nonetheless, the transfer has taken place.”

II. Methodology of Evidence Collection A forensic scientist must be meticulous in their work. First, he or she must observe general characteristics

of the evidence then observe more specific features. Second, they must use deductive reasoning to make

a determination as to the significance of the evidence.

Third, they must evaluate and verify all evidence; linking evidence to a crime and to the suspects by identifying and comparing relevant material.

Finally, a forensic scientist must come up with a theory or opinion that is able to stand up to scientific and legal scrutiny.

III. Federal Rules of Evidence

Evidence must be relevant; it must prove something (be

probative), and must address the issue

of the particular crime (be material).

Evidence is admissible if it is reliable and the person who presents it is believable and competent. Hearsay is inadmissible

IV. Evidence

Evidence can be divided into 2 general types: Testimonial evidence

Example: A witness pointing to someone in the courtroom, saying, “That’s the guy I saw robbing the grocery store.”

Physical evidence Any tangible thing, large or small.

V. Testimonial Evidence

Eyewitness accounts can provide important evidence leading to the arrest of a criminal

Juries are heavily influenced by eyewitness identification.

How accurate are eyewitnesses?

VI. Perceptual Errors by Eyewitnesses? Eyewitness errors can be made for many

reasons: The crime scene may have been too dark The encounter too brief The presence of a weapon may have diverted

their attention Stress and fear can sharpen focus for some,

or confuse others Time between the crime and questioning can

affect what he/she remembers

VII. Points to consider about eyewitness reliability…. The type of crime and how the eyewitness saw it Certain types of witnesses are better than others Interviewing techniques can make a difference Whether the witness knew the accused Whether the witness had already identified the

defendant or someone else.“Most cases of an innocent person being convicted happened because

of a mistaken eyewitness account.”

VIII. Physical Evidence

The Forensic Scientist is most interested in Physical evidence. more reliable than testimonial evidence

Most evidence does not prove a fact and is called indirect evidence.

Circumstantial Evidence implies a fact or event without actually proving it. the more there is, the greater weight it carries Ex: a blonde hair in the hand of a murder victim with

black hair. The hair is circumstantial because there are many blondes, but the observations limit the pool of suspects.

IX. Common Types of Physical Evidence Drugs and toxic substances Paints Gunshot residues Firearms and ammunition Impressions (shoe, bite) Fibers Soil Glass Blood and body fluids Hair Tissues Pollen Feathers, teeth, and bones Tool marks Documents Plastics, resins, rubber material

Most often, a questioned or unknown sample is compared to a known or control sample.

X. The Value of Physical Evidence

Physical Evidence is valuable for several reasons: Can prove that a crime has been committed Can back up witness testimony, or prove it

false Can link a suspect with the victim or crime

scene Can determine the identity of people

associated with the crime Can allow investigators to reconstruct a crime.

XI. Significance of Physical Evidence: Individual vs. Class The best evidence is anything that can be linked to a unique,

single, specific source. This is called Individual Evidence. Ex- fingerprints, handwriting, DNA, tool marks, broken

glass

If the object has characteristics common to a group of similar objects, not to one specifically, it is called Class Evidence.

Ex- blue jeans, shoes, blood types

A way to increase the probative value of evidence A way to increase the probative value of evidence is to collect as many different types of objects as is to collect as many different types of objects as possible to link the suspect to the crime or the possible to link the suspect to the crime or the victim.victim.

XII. Trace Evidence

Trace evidence is physical evidence found in small amounts at a crime scene.

Can be used to indicate transfer from one source to another, and therefore indicate contact between individuals or objects. Often microscopic analysis is needed

Ex- hair, fiber, paint chips, body fluids, stains powders, explosive residue, glass particles, soil

Microanalysis and Examination of Trace Evidence

Ch 16

Pgs 315-324

XIII. Introduction – “What is Microanalysis?” Microanalysis is the application

of a microscope and microscopical techniques to the observation, collection, and analysis of “microevidence”. Deals with samples that range

from milligrams to micrograms

XIV. Purpose of Trace Examination

To determine whether an association of persons, places, and things can be established and the strength of that association.

Comparison of materials found

Samples usually sent to trace.....

Fiber Hair and fur Soils Paint Serology (blood) Controlled

substances “Mystery Powders” Glass GSR

XV. Instruments of Microanalysis

Compound Light Microscope (CLM) Stereo Binocular Microscope Polarized Light Microscope (PLM) Scanning Electron Microscope (SEM) Energy Dispersive X-ray Spectroscope (EDS)

XVI. Stereo Binocular Microscope

Most common type of instrument in the laboratory

Employed in the preliminary evaluation of evidence for the location and recovery of microscopic particles and materials

Constructed by two similar but separate optical microscope for observation by each eye simultaneously. Views are separated by a small angle

(<15°)so each eye sees the subject from a different perspective.

XVII. Compound Binocular Microscope

Similar to what you know... Both eyes see the same image Employs transmitted, bright field

illumination Magnification range of 25-1200x Allows observer to view

evidence in high detail Has a scale calibrated with a

stage micrometer

XVIII. Polarized Light Microscope (PLM)

Is a Compound Light Microscope fitted with two polarizing elements, (the polarizer, and the analyzer)

Polaroid Allows scientist to

improve image contrast and quality,

Image from a PLM

XIX. Comparison Microscopes

Dissecting microscope, is a device used to analyze side-by-side specimens

Observer can see two image simultaneously

Large tool marks, fabrics, ballistics, hairs and fibers,

XX. Scanning Electron Microscope (SEM) Permits the viewing of

samples at much greater magnification and resolution (10-100,000)

Electron beam generated a thermionic source is accelerated by 10,000 to 30,000 electron volts.

Beam is focused by the use of electromagnetic lenses to a small bean spot and swept over the sample.

SEM images

XXI. Energy Dispersive X-ray Spectroscope (EDS)

Energy dispersive x-ray spectroscopy (EDS) is a chemical microanalysis technique performed in conjunction with a scanning electron microscope (SEM) .

The technique utilizes x-rays that are emitted from the sample during bombardment by the electron beam to characterize the elemental composition of the analyzed volume.

Features or phases as small as about 1µm can be analyzed.

EDS.... The EDS x-ray detector

measures the number of emitted x-rays versus their energy.

The energy of the x-ray is characteristic of the element from which the x-ray was emitted.

A spectrum of the energy versus relative counts of the detected x-rays is obtained and evaluated for determinations of the elements present in the sampled volume.