Witness Testimony and Credibility
Core concepts
A witness must be competent to testify: the witness must have personal knowledge of the matter and must take an oath or affirmation. Children and persons with cognitive impairments may testify if they understand the duty to tell the truth. Lay witnesses may give opinion testimony only if it is rationally based on perception and helpful to a clear understanding of the testimony; experts may give opinion testimony where specialised knowledge will assist the tribunal, provided the basis and reasoning are disclosed.
Credibility is the heart of this domain. Understand direct examination, cross-examination (leading questions permitted), redirect, and the scope of cross-examination. Learn the difference between impeachment by contradiction, by prior inconsistent statement (with the opportunity to explain), by bias, by interest, by conviction, and by attack on character for truthfulness. Prior inconsistent statements may also be substantive evidence if given under oath at a prior proceeding. Rehabilitation follows impeachment - good reputation for truthfulness, or consistent statements offered to rebut a charge of recent fabrication.
Traps
Questions often conflate impeachment with substantive admissibility. A prior inconsistent statement used only to impeach does not prove its contents. Watch for questions about refreshing memory: a writing used to refresh may be inspected by the opposing party, but it does not become an exhibit. Also watch for the collateral-matter rule limiting extrinsic proof.
Worked example
A witness testifies at trial that the light was green, but a signed statement says it was red. The statement can be used to impeach if the witness is given an opportunity to explain; it becomes substantive evidence only if the prior statement was under oath at a proceeding.
Rule of thumb
Ask whether the credibility attack is offered to impeach or to prove a fact - the answer determines the instruction and the limits.