Contents 4
Sufficient consensus
Search engine marketing covers paid placement on result pages. Pay-per-click campaigns run through an ad auction where bid and quality score determine placement and price, and the landing page carries the conversion.
Campaigns are organized in keyword groups, match types, budgets and reporting.
If every paid click were a new visitor, switching the ads off would lose all of those clicks. On eBay's brand terms nearly all of them arrived through the free listings instead.
| eBay paid search experiments | Result |
|---|---|
| Brand keyword ads switched off: click traffic retained | 99.5 % |
| Non-brand ads switched off: paid clicks that were newly acquired | 42 % |
| Return on investment, simple regression estimate | 4,173 % |
| Return on investment, experimental estimate | −63 % |
Blake, T., Nosko, C. and Tadelis, S., Consumer heterogeneity and paid search effectiveness: A large scale field experiment, NBER Working Paper 20171, 2014. Controlled experiments on eBay's paid search advertising in the U.S.
What the result pages leave out
These belong to the subject. People ask about them. They are missing from the agreed coverage.
- The second-price mechanism. The generalized second-price rules determine what is actually paid.
- Bid shading. Automated bidding features hide the price reduction algorithms from the people running the campaign.
- Cannibalization. Paid and organic search are analyzed in separate silos, so paid clicks that would have arrived for free stay invisible.
- Zero-click search. Reporting favours referral clicks over the far larger group of searchers who were satisfied on the result page itself.
- Click fraud measurement. Platform documentation points at its own filters. Independent measurement of invalid traffic is rarely part of the offer.
- Goodhart's law. Optimizing the quality score itself inflates the metric and degrades the return it was supposed to represent.
- Answer engines and generative results. A growing share of answers is synthesized above the blue links.
- Customer lifetime value. Campaign evaluation stops at the acquisition transaction and never reaches long-term profitability per customer.
What people actually want to know
- How much of my paid traffic would have come to me anyway?
- What do I actually pay for the click, and who decided that number?
- How much of my budget is spent on impressions that no human being ever saw?
- What happens to my campaign when the answer appears above the first ad?
Paid search is a fast instrument with a slow reckoning. The reckoning is the difference between acquired transactions and retained customers.
Last revised 2026-09-16.