No matter whether you are the patent owner or petitioner in an IPR, expert witness testimony is an important aspect of building your case. The Board relies on expert witnesses to provide insight into the often highly technical aspects of a given case, and will inevitably choose to give more weight to the testimony of one side’s witness over the other. This can be especially true when the testimony of the opposing experts is in direct conflict. So how does the Board choose which expert testimony to give more weight? Sometimes it’s a question of timing, as in the recent decision in Mylan Pharmaceuticals Inc. v. AstraZeneca (IPR2015-01340).