From the 2009 NAS report at page S-8:
Daubert and its progeny have engendered confusion and controversy. In particular, judicial
dispositions of Daubert-type questions in criminal cases have been criticized by some lawyers and
scholars who thought that the Supreme Court’s decision would be applied more rigorously.19 If one
focuses solely on reported federal appellate decisions, the picture is not appealing to those who have
preferred a more rigorous application of Daubert. Federal appellate courts have not with any
consistency or clarity imposed standards ensuring the application of scientifically valid reasoning
and reliable methodology in criminal cases involving Daubert questions. This is not really
surprising, however. The Supreme Court itself described the Daubert standard as “flexible.” This
means that, beyond questions of relevance, Daubert offers appellate courts no clear substantive
standard by which to review decisions by trial courts. As a result, trial judges exercise great
discretion in deciding whether to admit or exclude expert testimony, and their judgments are subject
only to a highly deferential “abuse of discretion” standard of review. Although it is difficult to get a
clear picture of how trial courts handle Daubert challenges, because many evidentiary rulings are
issued without a published opinion and without an appeal, the vast majority of the reported opinions
in criminal cases indicate that trial judges rarely exclude or restrict expert testimony offered by
prosecutors; most reported opinions also indicate that appellate courts routinely deny appeals
contesting trial court decisions admitting forensic evidence against criminal defendants.


The situation appears to be very different in civil cases. Plaintiffs and defendants, equally,
are more likely to have access to expert witnesses in civil cases, while prosecutors usually have an
advantage over most defendants in offering expert testimony in criminal cases. And, ironically, the
appellate courts appear to be more willing to second-guess trial court judgments on the admissibility
of purported scientific evidence in civil cases than in criminal cases.
But the reported opinions do not offer in any way a complete sample of federal trial court dispositions of Daubert-type questions in criminal cases.