David Brady is a Davies Family Senior Fellow at the Hoover Institution; the Bowen H. and Janice Arthur McCoy Professor of Political Science and Leadership Values in the Stanford Graduate School of Business; and a professor of political science at Stanford. Douglas Rivers is a senior fellow at the Hoover Institution and a professor of political science at Stanford University. An expert on survey research, he is also chief scientist for YouGov PLC, an international, Internet-based polling firm. Brady and Rivers discuss polling and how to measure change over time.