What is quality score?

Google computes Quality Score per keyword, from three components reported at a Below average, Average or Above average level rather than as a raw number: expected click through rate, ad relevance and landing page experience. Google is explicit that the score itself is not an input in the ad auction and is not used at auction time to determine Ad Rank. It describes Quality Score instead as a diagnostic tool for comparing your ad quality with other advertisers. Google does separately say that higher quality ads can lead to lower prices and better ad positions, but that is a statement about ad quality, not about the reported 1-10 column being what the auction consumes. Keeping those two apart is the whole of reading this column correctly.

The most common misuse of Quality Score is treating it like a flow metric: summing it, averaging it across a reporting period, or plotting it as a trend the way you would spend or conversions. Google's guidance is direct here: Quality Score is not a key performance indicator and should not be optimised or aggregated with the rest of your data. It is better read as a snapshot of current estimated quality on a keyword, which is the sense in which that instruction not to aggregate should be taken.