That these requirements are embodied in Rule 702 is not surprising.
Unlike an ordinary witness, see Rule 701, an expert is permitted wide
latitude to offer opinions, including those that are not based on
first-hand knowledge or observation.  See Rules 702 and 703.
Presumably, this relaxation of the usual requirement of first-hand
knowledge-a rule which represents -a `most pervasive manifestation' of
the common law insistence upon `the most reliable sources of
information,'- Advisory Committee's Notes on Fed. Rule Evid. 602
(citation omitted)-is premised on an assumption that the expert's
opinion will have a reliable basis in the knowledge and experience of
his discipline.
                            C
  Faced with a proffer of expert scientific testimony, then, the trial
judge must determine at the outset, pursuant to Rule 104(a), whether
the expert is proposing to testify to (1) scientific knowledge that
(2) will assist the trier of fact to understand or determine a fact in
issue.  This entails a preliminary assessment of whether the reasoning
or methodology underlying the testimony is scientifically valid and of
whether that reasoning or methodology properly can be applied to the
facts in issue.  We are confident that federal judges possess the
capacity to undertake this review. Many factors will bear on the
inquiry, and we do not presume to set out a definitive checklist or
test.  But some general observations are appropriate.
  Ordinarily, a key question to be answered in determining whether a
theory or technique is scientific knowledge that will assist the trier
of fact will be whether it can be (and has been) tested.  -Scientific
methodology today is based on generating hypotheses and testing them
to see if they can be falsified; indeed, this methodology is what
distinguishes science from other fields of human inquiry.-  Green, at
645.  See also C. Hempel, Philosophy of Natural Science 49 (1966) (-
[T]he statements constituting a scientific explanation must be capable
of empirical test-); K. Popper, Conjectures and Refutations: The
Growth of Scientific Knowledge 37 (5th ed. 1989) (-[T]he criterion of
the scientific status of a theory is its falsifiability, or
refutability, or testability-).
  Another pertinent consideration is whether the theory or technique
has been subjected to peer review and publication. Publication (which
is but one element of peer review) is not a sine qua non of
admissibility; it does not necessarily correlate with reliability, see
S. Jasanoff, The Fifth Branch: Science Advisors as Policymakers 61-76
(1990), and in some instances well-grounded but innovative theories
will not have been published, see Horrobin, The Philosophical Basis of
Peer Review and the Suppression of Innovation, 263 J. Am. Med. Assn.
1438 (1990). Some propositions, moreover, are too particular, too new,
or of too limited interest to be published.  But submission to the
scrutiny of the scientific community is a component of -good science,-
in part because it increases the likelihood that substantive flaws in
methodology will be detected.  See J. Ziman, Reliable Knowledge: An
Exploration of the Grounds for Belief in Science 130-133 (1978);
Relman and Angell, How Good Is Peer Review?, 321 New Eng. J. Med. 827
(1989).  The fact of publication (or lack thereof) in a peer-reviewed
journal thus will be a relevant, though not dispositive, consideration
in assessing the scientific validity of a particular technique or
methodology on which an opinion is premised.
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