Frequency measures the average number of exposures received by the portion of the defined population that was “reached” (i.e., received at least one exposure to the advertising or campaign) being assessed. 
Frequency measures how strongly an advertisement is concentrated on a given population. Net reach and frequency are important concepts in describing an advertising campaign; they separate total impressions into the number of people reached and the average frequency with which those individuals are exposed to advertising. A campaign with high net reach and low frequency runs the danger of being lost in a noisy environment. A campaign with low net reach but high frequency can overexpose some audiences and miss others entirely. Reach and frequency metrics help managers adjust their advertising media plans to fit their marketing strategies.
Frequency is calculated by dividing gross impressions by reach. Frequency is equal to the average number of exposures received by individuals who have been exposed to at least one impression of the advertising in question. Frequency is calculated only among individuals who have been exposed to this advertising.
Average frequency (#): The average number of impressions reached per individual.
Impressions (#) = Reach (#) x Frequency (#)
- ^ Farris, Paul W.; Neil T. Bendle; Phillip E. Pfeifer; and David J. Reibstein (2010). Marketing Metrics: The Definitive Guide to Measuring Marketing Performance (Second Edition). Upper Saddle River, New Jersey: Pearson Education, Inc.