Measurement
Reach and frequency
Reach and frequency are paired metrics: reach is how many different people saw an ad, and frequency is the average number of times each of those people saw it.
These two numbers describe how a campaign was spread. Reach counts unique people or households exposed at least once; frequency is total impressions divided by reach. Together they tell you whether you reached many people a few times or a few people many times.
For example, 10 million impressions could mean 5 million people seeing the ad twice, or 1 million seeing it ten times. The right balance depends on the goal: awareness campaigns want broad reach; complex messages may need higher frequency; too much frequency wastes money and annoys people, which is why frequency capping exists.
Measuring unique reach across devices and platforms is hard, because the same person may use a phone, laptop and TV. Identity signals, panels and modelled approaches such as household IDs help. Invalid traffic also distorts both numbers: a bot that sees an ad a hundred times inflates frequency, and many bots inflate reach. In TV planning, reach and frequency combine into gross rating points.
Think of it like this
Reach is how many different friends you told a piece of news; frequency is how many times, on average, each of them heard it from you.
An example
A campaign in the Netherlands delivers 12 million impressions and reaches 3 million unique people, an average frequency of 4. Capping frequency at 3 frees budget to reach an extra 800,000 people.
Related terms
Reach
Reach is the number (or percentage) of unique people or households who saw an ad at least once during a campaign, as opposed to the total number of impressions.
Frequency capping
Frequency capping is a setting that limits how many times the same person, device or household sees a particular ad within a set period, such as three times per day.
Gross rating point (GRP)
A gross rating point (GRP) measures advertising weight: reach as a percentage of a target population multiplied by average frequency, so 1 GRP equals impressions equal to 1% of that population.
Household ID
A household ID is an identifier representing a home and the devices in it, rather than one person, used mainly in CTV and addressable TV for targeting, frequency capping and measurement.
Impression
An impression is one instance of an ad being served and counted as displayed to a user; it is the basic unit that most online advertising is bought and sold by.
Sources: MRC standards and guidelines