Every year, millions of people stream through museums--young people and old people--people with varying degrees of education, people alone and in groups. How can museums best serve this diverse audience? One kind of service that museums try to provide is education. Unlike schools, which have age-graded classes and compulsory attendance, museums come face to face with the realities of "free-choice" learning. These realities ensure that predicting what and how visitors learn--let alone if they learn--will be very difficult. One useful index of visitor behavior in a museum becomes an important