Due to the broad discretionary power of the court in deciding who qualifies as an expert witness, once the individual qualifies, his or her testimony will be admissible even if there is a lack of sufficient factual support. Where a retained testifying expert does not have sufficient facts to support his or her testimony, only the weight given to the testimony will be affected, not its admissibility. Hypothetical questions may also be allowed if all factual items included in the hypothetical situation have been introduced in evidence before the questions are posed. However, the hypothetical question need not include any particular number of facts, and need not cover all facts alleged in the case. The questioning attorney may select from the facts in the case as a whole and obtain an opinion on that basis.
Limitations imposed on cross-examination are within the discretion of the trial judge.
The justification for such broad latitude is that cross-examination is the principal safeguard against errant expert testimony. Both examination and cross-examination are important to the main goal of trials, which is the search for truth. This search using expert testimony is facilitated by the opportunity given to the attorneys to provide evidence which will help the jury in weighing the credibility of the witness. Examination and cross-examination are indispensable in bringing elements of witness credibility to light and for the benefit of the jury. The Federal Rules of Evidence, Rule 608 provides guidelines for attacking witness credibility.