Unavailability as a witness includes situations in which the declarant is absent from the hearing and the proponent of his/her statement has been unable to procure his/her attendance by process or other reasonable means. The four factors bearing on the psychological unavailability of a witness are the probability of psychological injury as a result of testifying, the degree of the anticipated injury, the expected duration of the injury, and whether the expected psychological injury is substantially greater than that of the average victim of a rape, kidnapping, or terrorist act.
Following is an example of a federal statute defining the term “unavailability as a witness.” Pursuant to USCS Fed Rules Evid R 804 (a), “"Unavailability as a witness" includes situations in which the declarant—
(1) is exempted by ruling of the court on the ground of privilege from testifying concerning the subject matter of the declarant's statement; or
(2) persists in refusing to testify concerning the subject matter of the declarant's statement despite an order of the court to do so; or
(3) testifies to a lack of memory of the subject matter of the declarant's statement; or
(4) is unable to be present or to testify at the hearing because of death or then existing physical or mental illness or infirmity; or
(5) is absent from the hearing and the proponent of a statement has been unable to procure the declarant's attendance (or in the case of a hearsay exception under subdivision (b)(2), (3), or (4), the declarant's attendance or testimony) by process or other reasonable means.”