The primary locus of this obligation is Rule 702, which clearly
contemplates some degree of regulation of the subjects and theories
about which an expert may testify.
-If scientific, technical, or other specialized knowledge will assist
the trier of fact to understand the evidence or to determine a fact in
issue- an expert -may testify thereto.-The subject of an expert's
testimony must be -scientific. . . knowledge.-  The adjective -
scientific- implies a grounding in the methods and procedures of
science. Similarly, the word -knowledge-connotes more than subjective
belief or unsupported speculation. The term -applies to any body of
known facts or to any body of ideas inferred from such facts or
accepted as truths on good grounds.-  Webster's Third New
International Dictionary 1252 (1986).  Of course, it would be
unreasonable to conclude that the subject of scientific testimony must
be -known- to a certainty; arguably, there are no certainties in
science.  See, e.g., Brief for Nicolaas Bloembergen et al. as Amici
Curiae 9 (-Indeed, scientists do not assert that they know what is
immutably `true'-they are committed to searching for new, temporary
theories to explain, as best they can, phenomena-); Brief for American
Association for the Advancement of Science and the National Academy of
Sciences as Amici Curiae 7-8 (-Science is not an encyclopedic body of
knowledge about the universe. Instead, it represents a process for
proposing and refining theoretical explanations about the world that
are subject to further testing and refinement-) (emphasis in
original).  But, in order to qualify as -scientific knowledge,- an
inference or assertion must be derived by the scientific method.
Proposed testimony must be supported by appropriate validation-i.e., -
good grounds,- based on what is known.  In short, the requirement that
an expert's testimony pertain to -scientific knowledge- establishes a
standard of evidentiary reliability.
  Rule 702 further requires that the evidence or testimony -assist the
trier of fact to understand the evidence or to determine a fact in
issue.-  This condition goes primarily to relevance.  -Expert
testimony which does not relate to any issue in the case is not
relevant and, ergo, non-helpful.-  3 Weinstein & Berger -702[02], p.
702-18.  See also United States v. Downing, 753 F. 2d 1224, 1242
(CA31985) (-An additional consideration under Rule 702-and another
aspect of relevancy-is whether expert testimony proffered in the case
is sufficiently tied to the facts of the case that it will aid the
jury in resolving a factual dispute-).  The consideration has been
aptly described by Judge Becker as one of -fit.-  Ibid.  -Fit-is not
always obvious, and scientific validity for one purpose is not
necessarily scientific validity for other, unrelated purposes. See
Starrs, Frye v. United States Restructured and Revitalized: A Proposal
to Amend Federal Evidence Rule 702, and 26 Jurimetrics J. 249, 258
(1986).  The study of the phases of the moon, for example, may provide
valid scientific -knowledge- about whether a certain night was dark,
and if darkness is a fact in issue, the knowledge will assist the
trier of fact.  However (absent creditable grounds supporting such a
link), evidence that the moon was full on a certain night will not
assist the trier of fact in determining whether an individual was
unusually likely to have behaved irrationally on that night.  Rule
702's -helpfulness- standard requires a valid scientific connection to
the pertinent inquiry as a precondition to admissibility.
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