For as long as I can remember, Google have had two methods of rotating your ads within a particular ad group. Optimized or Rotate. When you setup a new campaign, by default Google set the ad rotation to Optimized. In the past, that meant that the ad with the better performing CTR would be shown more often.
Under certain circumstances this might work well for you. A good CTR, however, is not always a good thing, especially if it is attracting general or broad clicks. For Google, all clicks are good.
Our company’s stance on this setting was to always select Rotate. That’s the option which “shows ads more evenly”. Then you need to pause, delete or create ads as the data pours in based on what you see.
Google have just introduced a new option which optimizes for conversions. At first glance this might sound like a good idea. You want conversions, right? The problem is that conversions or CTR can never be read in isolation. You need a balanced approach with these two performance indicators. I have seen many situations where an ad results in a great CTR, but produces a low number of conversions. The opposite holds true as well. You may have an ad with a high conversion rate that has a low CTR, ultimately resulting in few conversions because no one clicks on the ad.
Google, why not make the ad rotation take both CTR and Conversions into account?