4-Step Process of Admissibility

Evidence is any fact, circumstance or object that is any part of the crime; it is a trace of human agency, the remnant of some phyical activity. Evidence comes in many different forms: a witness's description of an observed act or event, a photograph, a video, a fingerprint, forensic testing results, or some opinion from an expert witness. No matter which of these forms the evidence takes, the admissibility of all evidence can be determined by following this 4-step process of admissibility:

1. the witness is testifying from a proper basis of knowledge;
2. the testimony is trustworthy according to a rule of evidence;
3. the evidence was lawfully obtained; and
4. relevant.

This casebook will be divided according to this 4-step process. 

First, the practitioner must establish that the witness has the proper basis of knowledge regarding the information they are called upon to deliver to the factfinder. The rules tell us that there are two proper bases of knowledge: personal experience or expert testimony. Before any witness can be called upon to recount anything they have seen or heard, the practitioner must establish that they did so personally. For example, that they personally saw the man in the mask run away from a crime scene, heard cries of distress from a robbery, or overheard their friend confess to murder. We commonly refer to this first type of witness as a "lay-witness" and their testimony as "lay-testimony." The second type of witness is referred to as an "expert witness." Before this type of witness can give jurors their expert opinion, they must be properly qualified as an expert witness based on the witness's training and experience. Only if the practitioner establishes that the witness is testifying from a proper basis of knowledge about the subject matter of the their testimony can the practitioner move to the second step in our process of admissibility.

In that second step, the testimony must be legally trustworthy according to a constellation of evidentiary rules. Consider three examples. When a witness has observed something for themselves, Rule 602 establishes that their observations are trustworthy enough to be considered by a jury; if a person is qualified as an expert and can formulate an opinion based on valid scientific principles, then Rule 702 establishes that the opinion is trustworthy enough to be considered by a jury; when a person claims that someone confessed a murder to them, Rule 803(24) establishes how this out of court statement may be considered trustworthy enough to be heard by a jury. Only when the practitioner has (1) established that the witness has the proper basis of knowledge and (2) has offered it according to a rule establishing that the matter is trustworthy can the practitioner move on to the third step. 

Even though the practitioner has established the proper basis of knowledge (step 1) and that the testimony is legally trustworthy (step 2), they next establish that the evidence was lawfully obtained. Thus, even if a police officer personally observed some contraband in a suspect's house, the practitioner must establish that the officer had the lawful authority to be there because of a search warrant or some exception to the warrant requirement. The same would apply to a suspect's confession: even if the officer personally observed the interview, and the defendant made statements that are legally trustworthy according to the hearsay rules, the practitioner must prove that the confession was made voluntarily and with the proper warnings or else the confession would be inadmissible. Only after the practitioner has established that the witness has the proper basis of knowledge (step 1) and that the testimony is legally trustworthy (step 2) and was lawfully obtained (step 3) can the practitioner move on to the last and final step, relevance.

In this fourth and final step, the practitioner must establish that the evidence is relevant. When establishing relevance, the practitioner must prove to the court that the evidence is "probative" in that it makes the existence of any fact of consequence in the trial more or less probable, and that the evidence is not "prejudicial," meaning the evidence does not suggest a decision to the jury on some improper bases. As we will see, relevance is often case-specific and fluid: what is relevant in one trial may not be relevant in another, or what is irrelevant at the beginning of a trial may be relevant by the end. But in all cases, for all types of evidence and all witnesses, the evidence must be relevant. If it is not relevant, it is not admissible.

All 4 steps must be satisfied for a piece of evidence to be admitted. If the practitioner fails at any one step along the way, there is no need to consider the next step. In this casebook, we will consider each step in order beginning with rules related to establishing that a witness has a proper basis of knowledge.