<b>Auction density and why pacing is harder than it looks</b>

<b>Auction density and why pacing is harder than it looks</b>

Budget pacing spends evenly over a flight instead of front-loading. The naive model assumes auction opportunities arrive uniformly. They don't, and the non-uniformity breaks simple pacing.

1. Auction density — the rate of eligible auctions for your target — varies by hour, day, and audience, sometimes 5–10x between trough and peak.
2. A pacing system that targets constant spend-per-hour will either starve during low-density hours (no inventory to buy) or get forced to overpay to hit its hourly number.
3. Worse, raising bids to hit a pacing target during a low-density window pollutes your clearing-price data, teaching the shading model that inventory clears higher than it really does.

The better model paces against <i>available opportunity</i>, spending more when good inventory is abundant and cheap and less when it's scarce and expensive — which usually means accepting an uneven hourly spend curve to get an even-quality outcome.

<b>Why it matters:</b> pacing and bidding are coupled, not independent. A pacing rule that forces spend during scarce hours quietly inflates your CPMs and corrupts the very model that's supposed to keep prices honest. Evaluate pacing on cost-per-outcome across the flight, not on how smooth the hourly spend line looks.