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INTRODUCTION
Virtually all cases involving scientific or technical issues, or even professional standards, require expert witnesses and expert testimony. The rules that have evolved and the standards that are required regarding expert testimony are by no means uniform, as they vary depending upon the jurisdiction. Juries are frequently asked to decide between the conflicting testimony of two or more expert witnesses, with little or no guidance from the court. As Judge Learned Hand pointed out more than a century ago, "how can the jury judge between two statements each founded upon an experience confessedly foreign in kind to their own? It is just because [jurors] are incompetent for such a task that the expert is necessary at all."1 The last half of the twentieth century saw an ever-growing debate about the standards that should be applied to expert testimony and the safeguards that should be applied to prevent abuse.2
At the outset, it must be remembered that there are three general areas of expert evidence: (1) pure opinion expert evidence, (2) scientific expert evidence, and (3) non-scientific expert evidence. The rules that govern the admission of each may vary, both in terms of the standards for admission in the first instance as well as the standard for reviewing decisions of the trial court. Different jurisdictions have formulated different tests for the admission of all types of expert evidence, and care must be taken to identify precisely what rules apply to each jurisdiction.
All too often courts avoid the issue of admissibility by stating, "it goes to the weight, not the admissibility," and then simply let everything in. The recent trend, however, has been toward a more careful scrutiny of expert evidence by trial courts. As the Fifth Circuit Court of Appeals so aptly put it in In re Air Crash Disaster at New Orleans, Louisiana:3
Basic policy questions that affect the very nature of a trial lie behind decisions to receive expert testimony. Under the Federal Rules of Evidence, experts not only explain evidence, but are themselves sources of evidence.
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... [W]e recognize the temptation [on the part of the trial judge] to answer objections to receipt of expert testimony with the shorthand remark that the jury will give it "the weight...