Gross rating points (GRPs) or target rating points (TRPs) are chiefly used to measure the performance of TV-based advertising campaigns, and are the sum of the TVRs of each commercial spot within the campaign. An ad campaign might require a certain number of GRPs among a particular demographic across the duration of the campaign. The GRP of a campaign is equal to the percentage of people who saw, multiplied by the average number of spots that these viewers saw. Targeted Rating Points are a refinement of GRPs to express the reach time frequency of only the most likely prospects. For example, if a campaign buys 150 GRPs for a television spot, but only half of that audience is actually in the market for the campaign's product, then the TRP would be stated as 75 to calculate the net effective buy [https://web.archive.org/web/20090611172153/http://forum.researchinfo.com/showthread.php?t=1217].