Telephone surveys typically involve calling people at home during the evening or on weekends to ask a series of questions. Over the years, securing respondent cooperation for telephone surveys has become more difficult, with the increased use of telemarketing and with more people screening their calls using answering machines. However, telephone surveys can be very useful when designed to control for demographic data and performed by skilled interviewers. They allow collection of a large body of data for a relatively low cost, and with a fairly high level of reliability.