First-price vs second-price: which bid strategy fits which inventory

First-price vs second-price: which bid strategy fits which inventory

The choice is not philosophical. It is mechanical, and it changes how you should set your bids.

1. Second-price auction: the winner pays one cent above the second-highest bid (your bid is a ceiling, not a charge). Optimal strategy is to bid your true value, because you rarely pay it.

2. First-price auction: the winner pays exactly what they bid. Bidding true value now overpays on every win.

3. Today nearly all open exchange supply is first-price after the 2017-2019 transition. But private marketplace deals and some networks still clear second-price, and a few run hybrid.

4. The error: running one flat bid logic across both. On second-price inventory you can bid aggressively to win, paying near the runner-up. Copy that bid into first-price and you pay your full aggressive number every time.

5. When to use each lever — on first-price supply, lean on bid shading (the DSP lowering your bid toward the expected clearing price) and tight pacing. On second-price supply, lean on raw bid value and let the auction mechanic protect you.

Why it matters: the same $4 bid means a $4 cost in first-price and maybe a $1.80 cost in second-price. If your DSP exposes auction type in log-level data, segment win-cost by it before you trust any blended cost figure.