Advertising
Frequency
The average number of times each reached person saw the ad over a period.
Frequency = impressions / reach
What makes it wrong.
The mechanism, not the warning.
An average hides the distribution. A frequency of three can mean everyone saw the ad three times, or that most saw it once and a small group saw it fifteen times. The second is where budget is actually being wasted, and it needs a distribution, not an average.
In detail.
Frequency is the first place to look when performance decays without anything having changed. A rising frequency on a fixed audience means the same people are being served more often, which reliably lowers response.
It is period-dependent by construction: the same campaign has a low weekly frequency and a high quarterly one.