Gross Rating Points (GRP) are the total number of in-market impressions delivered by an Out of Home (OOH) or outdoor billboard campaign schedule expressed as a percentage of a market population and is a measure of advertising impact. A one rating point represents impressions equal to 1% of the market population. In the calculation of GRPs, total impressions must first be reduced to the in-market of individuals who live in the defined market and are part of that market’s population. You calculate it as a percent of the target market reached multiplied by the exposure frequency. Thus, if you get advertise to 30% of the target market and give them 4 exposures, your campaign would have 120 GRP.
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