A fourth influential area of early media research, marketing research, developed when advertisers and product companies began conducting surveys on consumer buying habits in the 1920s. The emergence of commercial radio led to the first ratings systems that measured how many people were listening on a given night. By the 1930s, radio networks, advertisers, large stations, and advertising agencies all subscribed to ratings services. However, compared with print media, whose circulation departments kept careful track of customers’ names and addresses, radio listeners were more difficult to trace. This problem precipitated the development of increasingly sophisticated marketing research methods to determine consumer preferences and media use, such as direct-mail diaries, television meters, phone surveys, telemarketing, and Internet tracking. In many instances, product companies looking for participation in their surveys paid consumers nominal amounts of money to take part in these studies.