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Ad unit quality is defined by our bot on base of several criteria: site content, length of domain name, errors in html code, rating on Alexa, etc. High quality ad units are those placed on the sites, that our bot liked best.

Newly created ad units usually have 0.125 quality by default; those on better domains might have 0.25. After moderation their quality might be fixed.

Ad units with quality lower than 0.125 are usually really low quality ones, and you probably don't want them in your campaign targeting.

Normally you don't need to change quality range for your campaign, Even a miserable change of quality range can cause dramatic reduce of available ad units. E.g. default range is 0.125 - 1 and you get several thousand ad units. You set 0.5 - 1 range and get only 30 ad units. This will probably worsen your campaign performance.
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