to be clipped to an individual's clothing. Radio stations encode an inaudible, unique signal as part of their broadcasts. The PPM "hears" this signal and records the station and the time spent listening. Such a device requires far less effort on the part of respondents. See also Single-source data References Sources Lotz, Amanda D. "The Television Will Be Revolutionized." New York University Press. Retrieved April 16, 2011. Stoddard Jr., Laurence R. 'The History of People Meters. How we got where we are(and why)', Journal of Advertising Research, Oct/Nov 87, Vol. 27 Issue 5. Lowry,Brian. Variety: August 16–22, 2004, Vol 395 Issue 13, p 14-16, 2p. External links http://onlinedatingpost.com/archives/2006/03/nielsen_tracking_box/ Blog post containing a picture of a "People meter" Television technology Audience measurement Broadcasting Market research 1986 introductions 1987 establishments in the United States British inventions American inventions pt:People meter