Google really doesn't trust the average Adwords user when it comes to split testing their ads. How do I know? In Adwords campaign settings, Google gives users two options for ad rotation:

• Optimize - 'Show better performing ads more often.' With this option, after a short amount of time Google takes the ad with the highest CTR and then gives it over 80% of the impressions, displaying it much more often than the other ad.
• Rotate - 'Show ads more evenly.' This tells Google to give each ad an equal number of impressions. Best for smart advertisers.

I recommend that as an advertiser you shouldn't listen to Google's recommendation to optimize your ads. Letting Google pick your best ads will kill your split tests before you collect enough data to truly declare a mathematical winner. Despite this, Google has always set the default for this setting at "Optimize," even going so far as to recommend it for new advertisers. Recently, as of August '09 Google updated their Adwords web interface to make it even harder to change this ad rotation option. In fact, if you're setting up your new campaign settings before you write any ads, as I'm sure many do, it's literally not an option for you. Here's a screenshot of the ad rotation setting for new, ad-less campaigns: So, if showing your ads evenly is the best way to run split tests to get fast results, why doesn't Google set this as the default? I speculate about this and write about two other settings you should go against Google's recommendation on for every new Adwords campaign you create in my post, "3 Adwords Settings You Must Change to Avoid Campaign Suicide." I recommend that as a smart advertiser you should set up your ads (or just create a test ad) and change this ad rotation setting to "Standard," giving all of your ads an even shot at becoming winners for your campaign.