DATA AND METHODS
This section details the data used in our analysis and our analytic approach. We start by describing how we selected federal district court opinions addressing challenges to expert evidence and how we extracted the salient information from them. We then provide an overview of the opinions in our sample and conclude by describing the methods used to analyze them.
We drew our sample of opinions from Westlaw's database of federal district court opinions. Figure 3.1 puts the Westlaw database into a broad perspective. Excluding prisoners' petitions, social security cases, and cases in which the federal government is attempting to recover debts,1 roughly 160,000 civil cases are filed in federal court each year. The proportion of cases that involve expert evidence is unknown, as is the proportion of expert evidence that is challenged. Roughly 150 opinions addressing challenges to expert evidence in civil cases appear in Westlaw's database of federal district court opinions each year.
Westlaw provides a readily accessible source of district court opinions that can be searched in electronic form, but its limitations should be understood. First, Westlaw contains only written opinions, as opposed to oral rulings on challenges to expert evidence from the beach. The proportion of opinions on challenges to expert evidence that are written versus oral is unknown. It seems likely, however, that exclusion of expert evidence is more likely to lead to an appealable issue and that judges thus may issue written opinions more frequently when they exclude challenged evidence than when they admit it The frequency with which evidence is excluded will likely be higher in the Westlaw database than for all opinions. This is not a problem for our analysis as long as there is no systematic change over time in the relative propensities of judges to issue written opinions when evidence is excluded versus when it is admitted. We return to this issue in Subsection 4.1.
Second, some written district court opinions are not reported to Westlaw. While Westlaw contains all officially published opinions (such as those appearing in the Federal Supplement) and a substantial number of “unreported” opinions submitted by judges, an unknown number of written opinions never appear in Westlaw. The proportion of opinions reported versus unreported____________________