Audience response systems, also known as "clickers", allow students in a classroom to answer poll questions to gauge understanding and engagement. Questions are displayed in PowerPoint and students respond using handheld devices. Responses are recorded and results can be analyzed. These systems encourage active learning over passive lectures. They provide instant feedback to students and teachers and create a collaborative experience. Questions can be used for teaching, assessment, evaluation, and revision. Audience response systems make large lectures more interactive and help improve motivation, attendance, and retention.