By contrast, FRE 702 states that "if scientific, technical, or other specialized knowledge will assist the trier of fact ;to understand the evidence or to determine a fact in issue, a witness — as qualified as an expert by knowledge, skill, experience, training, or education — may testify thereto in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case." As indicated, however, the rule concerning expert witnesses may vary from state to state.

Federal court jury instructions advise juries concerning expert witnesses as follows:

"The rules of evidence ordinarily do not permit witnesses to testify as or conclusions. An exception to this rule exists as to opinions to those whom we call 'expert witnesses.' Witnesses who, by education and experience, have become expert in some art, science, profession, or calling, may state an opinion as to relevant and material matter, in which they profess to be expert, and may also state their reasons for the opinion.

"You should consider each expert opinion received in evidence in this case, and give it such weight as you may think it deserves. If you should decide that the opinion of an expert witness is not based upon sufficient education and experience, or if you should conclude that the reasons given in support of the opinion are not sound, or that the opinion is outweighed by other evidence, you may disregard the opinion entirely."

(Source: Devitt and Blackmar, Federal Jury Practice and Instructions, 3rd ed., Vol.1 [West Publishing Company, 1977], § 15.22, p. 482.)

Many state courts offer similar typical expert witness jury instructions that could read as follows:

You have heard evidence in this case from witnesses who testified as experts. The law allows an expert to express opinions on subjects involving their special knowledge, training, skill, experience or research. You shall determine what weight, if any, should be given such testimony, as with any other witness.

A witness who is qualified by the court as an expert in a particular field may assist the judge or jury in understanding a method or technology at issue, interpreting results of scientific tests, or offering opinions based upon the evidence in the case. Testifying experts are not required to conduct the tests on which they base their opinions. However, they must possess the required "scientific, technical or specialized knowledge" to be qualified by the court as an expert in their particular field in order to review relevant materials from the case and/or give relevant opinions.

Experts who work for federal, state, local and private laboratories may have written standard operating procedures (SOPs), which outline all steps of each scientific analysis the laboratory performs. These should be followed whenever possible. In instances where a deviation from the SOP is necessary, a thorough explanation should be documented and immediately brought to the attorney's attention.

Generally, a forensic expert's role begins by:

• Receiving information and evidence in a case for testing.
• Examining or evaluating a case.
• Receiving an opposing expert's analytical report.
• Anticipating and preparing for cross-examination.

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