A cross-SSP frequency cap stopped 28% of impressions hitting fatigued users

A cross-SSP frequency cap stopped 28% of impressions hitting fatigued users

Frequency cap (a limit on how many times one user sees a campaign). A buyer capped at 3 per day per SSP but tracked users separately on each path, so the real frequency ran far higher.

1. They unified user IDs across SSPs in log-level data and recomputed true daily frequency per user.
— 28% of impressions went to users already at 7-plus exposures that day, well past the intended 3.
2. The per-SSP caps each held, but four SSPs each delivered 3 to the same user, summing to 12.
3. They moved frequency capping to the DSP layer, enforced once across all paths.

Evidence: those 28% of wasted impressions were reallocated to fresh users. At flat spend, unique reach rose 19% and conversion rate per impression climbed 11%, since budget stopped pounding fatigued users with no marginal lift.

Why it matters: a frequency cap enforced per SSP is not a frequency cap. The same user arrives through every path. Capping must happen at the DSP, where all paths converge, or the real exposure is the sum of every per-path limit.