A major ongoing story in litigation of many kinds lies in the rules on admission of scientific evidence. The practical impact of these rules is that in a large range of cases involving scientific evidence, courts now require "Daubert hearings" that test expert testimony before trials are held. The court's opinion captures several themes common to cases excluding expert opinion on scientific causation. The court's opinion reflects the difficulty in aligning law and science in what it called "scientifically controversial" areas such as litigation on breast implants. Scientific studies are usually fair game for argument in litigation, but occasionally courts will give special credit to a particular study. The role of experts often plays a pivotal part in legal controversies involving scientific evidence. Courts have presented long lists of factors that may be considered in making this judgment. The decisions of many courts apply various factors outlined in the Daubert case.