When you create an AdWords campaign, you can edit the Ad Serving setting to either “rotate” or “optimize” the showing of the Ad.
I’ve always read, and generally practiced, using the rotate option to help me refine better and better Ads. The thought being that if Google rotated between two Ads evenly, you would begin to see which one is written better by a higher CTR and possibly conversion rate.
BUT…this doesn’t actually mean that your two Ads are in essence being given equal impression opportunities.
Why is this? Several variables go into whether your Ad will get shown (an impression), one of those variables being Ad Rank. Ad rank is a ‘hidden’ score that is looked at prior to deciding whether your Ad will be shown…and obviously your two Ads may score slightly different, one being lower. Based on the dynamic nature of the Ad serving process, picking “rotate” may mean that you are giving up on potential impressions, since your top Ad (Highest Ad Rank) is being considered at that moment.
Ad Rank can vary from ad to ad, because
* Different ads cause different CTR with the same keywords,
* Quality Score (QS) is largely determined by CTR, and
* Ad Rank is the product of Bid * QS…
…so every ad has a different Ad Rank.