This free online video program "shows how to improve the accuracy of a survey by using stratified random sampling and how to avoid sampling errors such as bias. While surveys are becoming increasingly important tools in shaping public policy, a 1936 Gallup poll provides a striking illustration of the perils of undercoverage." This individual video is accessed by scrolling down to the "Individual Program Descriptions - 14. Samples and Surveys" and click the "VOD" icon at the top-right of the description.