At the local level, Nielsen measures television audiences using different methodologies depending on the market.
In the top markets (ranked 1-25), local people meters (LPM) are used to electronically capture both tuning and viewing. The LPM is considered the standard of measurement in the US today. See "Local People Meter Primer" to understand the advantages of LPMs in audience measurement.
In the next set of markets (ranked 26-56), set meters are used to capture set tuning continuously all year, while diaries are still mailed out during sweeps to capture demographics (who is watching). These two methods (what and who) are joined together to develop audience estimates in these markets. See "Buying in Meter-Diary Markets" for more information.
In the smallest markets (currently markets ranked above #57) diaries are sent out four times a year during "sweep" months. The broadcasters tend to schedule premieres, specials and original programming during these "sweep" periods, and this "hyped" data is used for buying and selling the rest of the year. Diaries are used to collect both viewing (who) and tuning (how many people) to programs and networks. The diary is the least accurate collection method. On average diaries under-report demo audiences 10-25%. See "Buying in a Diary-only Market" for more information.
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