We have conducted an empirical study of every reported doctrine of equivalents decision in both the Federal Circuit and the district courts during three periods - one before the Federal Circuit's 2000 Festo opinion, one after that opinion but before the Supreme Court's 2002 opinion, and a third after the Supreme Court's opinion. Two broader findings stand out. First, the multiple changes in the doctrine of equivalents rules over the last ten years have had surprisingly little effect on the actual outcome of doctrine of equivalents cases, and even less effect on the subset of cases dealing directly with prosecution history estoppel. Indeed, to the extent there is any relationship it is an inverse one - patentees did better under less patent-friendly rules. The attention everyone has paid to Festo as changing the value of patent rights therefore seems to have been largely wasted from a practical perspective.
The second finding is even more significant: the reason the Festo changes had so little effect seems to be that the doctrine of equivalents was already near death by the late 1990s. Even under the permissive doctrine of equivalents rules in place before 2000, while everyone was focused on the doctrine of equivalents, equivalents claims usually failed, most often on summary judgment. That became even more true after 2000, and the Supreme Court's 2002 decision didn't change the trend. In fact, district courts are more likely to reject doctrine of equivalents claims today than ever before.
This left us with a bit of a puzzle: what killed the doctrine of equivalents in the 1990s? We suspected the answer was the growth of claim construction Markman hearings after the Supreme Court's 1996 decision in that case. Once courts were construing claims as a matter of law pre-trial, and finding themselves in a position to resolve virtually all infringement issues on summary judgment, they were naturally inclined to decide the doctrine of equivalents issues too. And a court that has just rejected a literal infringement argument - the only courts likely to spend much time thinking about equivalents issues pre-trial - is unlikely to undo the work of claim construction by sending the issue of infringement by equivalents to the jury. To test this hypothesis, we constructed a fourth dataset, including cases decided in the 1993 to 1995 timeframe. That data bears out our hypothesis. The doctrine of equivalents was alive and well before Markman, but has been in decline ever since.