Admissibility

Evidence: The Concept of "Admissibility"
Evidence comes in four basic forms:
Demonstrative evidence
Documentary evidence
Real evidence
Testimonial evidence
Some rules of evidence apply to all four types and some rules apply to one or two of them. All of these forms of evidence must be admissible, though, before they can be considered as probative of an issue in a trial.
Basically, if evidence is to be admitted at court, it must be relevant, material, and competent. To be considered relevant, it must have some reasonable tendency to help prove or disprove some fact. It need not make the fact certain, but at least it must tend to increase or decrease the likelihood of some fact. Once admitted as relevant evidence, the finder of fact (judge or jury) will determine the appropriate weight to give a particular piece of evidence. A given piece of evidence is considered material if it is offered to prove a fact that is in dispute in a case. Competent evidence is that evidence that accords with certain traditional notions of reliability. Courts are gradually diminishing the competency rules of evidence by making them issues related to the weight of evidence.