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How does Google choose the ad displayed in search result amongst multiple ads in an ad group?
edited June 2016
Is this selection random or can we make any changes in AdWords or Optmyzr to show ads which perform better?
Google uses the ad rotation setting you have selected for the campaign. By default they will show the ad with the best CTR more often but you can also choose to give preference to the ad that leads to more conversions, or to rotate the ads evenly.
The 4 available settings are described here:
For using Optmyzr's A/B Ad Testing Tool, we recommend using one of the 2 settings for 'even rotation'.