Quality Score

Google gives each keyword in a Search campaign a score from 1 to 10. It is built from three components, each rated above average, average or below average: expected click-through rate, ad relevance and landing page experience. The comparison is with other advertisers whose ads showed for the same search over the last 90 days.

Google is clear that Quality Score is a diagnostic tool. It is not itself an input to the auction and is not meant to be treated as a target. The live auction uses fresher, more detailed quality signals inside Ad Rank. But the score is a useful window into them: better quality generally means a better position at a lower price.

This matters to arbitrage on both sides. In Search-to-search, the arbitrageur is the advertiser, and a thin or ad-heavy lander earns a poor landing page rating and a higher CPC, squeezing the spread. On the selling side, the ads that appear in a feed were ranked partly on quality, so their relevance affects Ad CTR.

It should not be confused with a traffic quality score, which rates a publisher's visitors, not an advertiser's ad.

Think of it like this

It is a school report with three subjects. The report does not decide the exam result, but it tells you which subject is dragging you down.