Metrics
Frequency capping
Frequency capping limits how many times one person can be served a campaign's ads within a period. It protects budget from being concentrated on a small, over-exposed group and protects the brand from the irritation that heavy repetition produces.
In practice
Caps are set per campaign, line or ad depending on platform, and work best when planned across the whole campaign rather than per platform - five platforms each capping at three still adds up to fifteen.
Watch delivery after tightening a cap: platforms respond by finding new people, which can raise reach and CPMs simultaneously.