A few years ago, my health sciences library held an open house aimed at preceptors. The event was a big success: Many people attended, and feedback was positive. Yet when we tried to repeat the event in subsequent years, attendance foundered. We wondered, “What had changed?” Had most of the interested preceptors attended the first time? Did our advertising reach a different audience the second time around. We thought we had done exactly the same thing as before, but clearly something was different. Unfortunately, we had no means to identify what that difference was.