The ad server makes the final ad-selection decision. Header bidding supplies competing offers before that decision, subject to the publisher’s timing and auction setup. (Prebid introduction and header-bidding role ↗)
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Following the auction order
In an illustrative auction with a 300-millisecond deadline, Partner A returns a usable bid after 150 milliseconds while Partner B responds after 350 milliseconds. Partner B’s bid cannot help that already-completed decision, even if its quoted price is higher. A timeout test asks whether waiting longer produces enough additional revenue to justify the extra delay.
What header bidding does not guarantee
Compare revenue across all assigned ad opportunities, including those that display no ad. A new bidder may add useful competition, duplicate existing demand or arrive too late. The number of configured bidders does not tell you which of those happened.
What a publisher can change independently
A publisher can test how long it waits for bids without changing the price it is willing to accept. Change one setting at a time. If both move together, a revenue difference gives you little evidence about which setting caused it.