It is important not to confuse the "relevance" and "weight" of evidence. While relevance describes the tendency of evidence to support logical inferences, the concept of "weight" relates to how "probative" or influential the evidence is. As the foregoing discussion demonstrates, the basic rule of admissibility requires relevance and materiality, but not weight. "Once evidence is found to be relevant [to a material issue] it is generally admissible and the [trier of fact] is left to decide how much weight to give a particular item of evidence."22Even though the law of evidence has relatively few "rules of reasoning" to assist decision-makers in weighing evidence, it is worthwhile examining the concept of weight closely and in a structured way. Thinking about weight this way will assist in preparing cross-examination, in making arguments, and in the judicial evaluation of evidence that is admitted. As will be seen shortly, this exercise is also useful in understanding the exclusionary discretion, about to be discussed, as well as several exclusionary rules of evidence described later in this chapter. This examination is not complex as the weight of evidence is simply a function of how believable and how informative the trier of fact considers it to be.
Assume a witness testifies that the robber with the gun was the person with a face tattoo of a stream of tears. If the trier of fact is concerned that the witness may be lying, or may be mistaken about this, the evidence may be given little if any weight. Believability affects weight.
When deciding whether evidence is believable, legal theory draws a helpful distinction between "credibility" and "reliability."23"Credibility" is about the honesty of the witness. Evidence showing that a witness has been corrupted, has a motive to mislead, or has a discreditable character will be relevant to credibility. For example, evidence that the witness identifying the robber was an accomplice who has made a deal with the police could cause a trier of fact to give the testimony little weight.
"Reliability" is the term used to describe the accuracy of evidence. It can relate to the accuracy of a scientific or forensic process, but when applied to witnesses, reliability captures the kinds of things that can cause even an...