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Admissibility of Evidence:
Relevance
Material - The evidence must relate to the case at hand. Previous criminal records are not material, and they can be prejudicial; as such, they are inadmissible.
Probativeness - The evidence must prove something. If the evidence, such as the possession of a poison by the accused, has no relevance to a death by strangulation, it is inadmissible.
Competence
Prejudice - Anything that unduly affects the trier-of-fact in an opinion either for or against the accused, is considered prejudicial. Prior criminal records, or inflammatory images may therefore be ruled inadmissible.
Constitutional Constraints - The 4th Amendment to the U.S. Constitution states the following:
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
As such evidence obtained without a warrant, without probable cause, and not listed in the warrant as the place, person, or thing to be seized, is therefore inadmissible, regardless of relevance.
Statutory Constraints - This is also called "Privileged Information," or information told to a person who cannot reveal it (such as lawyers or clergy).
Hearsay - Hearsay is a statement made outside of court by a person who was not under oath when the statement was made, but is then being used as proof that the statement was truth.
Some notes on important cases dealing with
the admissibility of evidence.
Expert Witness:
Forensic Evidence relies upon the testimony of expert witnesses.
Expert witnesses are judged to be expert only by the Judge her/himself. In order to be so judged, it is necessary to have the appropriate training and experience (which need not be a Ph.D.), as when a mechanic gives expert testimony regarding a car involved in an accident.A non-expert witness is called a lay witness.
Differences between an expert and a lay witness:
1. An Expert Witness must be qualified as an expert every time she/he testifies in court.
2. An expert witness is permitted to offer opinions, whereas a lay witness generally cannot.
Expert Testimony:
Lab Reports:
Although lab reports are technically an example of hearsay, as the report itself cannot be cross-examined. There are, however, exceptions: in some states both sides can agree to admit it (stipulate), some states require the author present, in some states it fits under a business records exemption.
Defense vs. Prosecution:
Innocent until proven guilty . . .
Reasonable Doubt - The Prosecution has the burden of proof. The defense has the responsibility to either exonerate the accused, or at least to introduce a reasonable doubt.
Beyond a Reasonable Doubt vs. No Doubt (The jurors must decide . . .). There is no precise definition, so it is thus in the hands of the jury.
Defense must provide a spirited defense, regardless of the guilt or innocence of the accused (or a mistrial can be called).
The Prosecution tries cases it can win (Winnability vs. The Desire to Seek Justice)
Discovery Motions:
The defense is required access to the witness list, as well as to the evidence, so that it may carry out its own evaluation of the evidence.
Chain of Custody & Evidence Contamination:
The chain of custody, if not carefully managed, not only increases the risk of contamination, but it also can introduce a reasonable doubt.