Today Google introduced AdWords Campaign Experiments (ACE), a tool designed to optimize AdWords accounts by letting advertisers test and measure changes to keywords, bids, ad groups, and placements.

"In the past, the most common way to measure how changes to your account affected your campaign performance was to measure how the campaign performed before and after you implemented your changes," says product marketing manager Courtney Lischke. "However, this type of analysis can often be complicated by events that occur during the test period, including holidays, weekends, or changes to end user or advertiser behavior."

"ACE allows you to perform faster, more accurate tests by executing your experimental campaign alongside your original campaign," says Lischke. "By performing this type of simultaneous split test, we can tell you precisely if your campaign changes produce statistically significant results."

Advertisers using ACE can measure things like the incremental impact of adding new keywords to their campaigns or changing keyword and ad group-level bids, the value of restructuring content campaigns to use more tightly themed ad groups, the change in volume by using different keywords match types, etc.

There are more how-to videos here, and an FAQ page here you can refer to for more info.

ACE is currently being tested, and Google is inviting U.S. advertisers to participate. Not all applicants will be accepted for beta testing, however.