Call it a test within a test: Google Inc. today began what it calls an experiment to see if marketers, retailers and consumers want to see Trusted Stores endorsements within paid search ads.

Trusted Stores is a customer service program that the search engine began testing last fall. Participating e-retailers can include the Google endorsement on their web pages. To take part, retailers must give Google average shipping times for products and enable it to collect other customer service data; Google then grades e-retailers on how well they resolve customer issues, and lets retailers display the grades. Consumers who complete a purchase with Trusted Store e-retailers also can opt in to receive Google’s free purchase protection program on their order. If there’s an issue with the order, Google will work with the e-retailer on the customer’s behalf to address the problem.

A couple of hundred retailers take part in Trusted Stores, says Tom Fallows, group product manager on Google’s commerce team. The new test includes about a dozen participants, he says, including online retailer Wayfair.com, the new brand name of CSN Stores LLC, No. 51 in the Internet Retailer Top 500 Guide.

He says the test will measure a variety of ways to put Trusted Stores information before the eyes of consumers viewing paid search ads. For instance, “Google Trusted Store” might appear below the main link for a retailer, or the Trusted Stores badge with information about how the retailer rates in certain areas (for instance, “99% of issues resolved in under two days.”) “Some versions will perform well, some won’t,” Fallows says. “We’ll pick the winners and riff off those.”

Fallows gives no timetable for the ongoing Trusted Stores test. He also won’t share details about how the Trusted Stores program in general is encouraging or affecting online shopping. “It’s been well received in terms of the key metrics we look at—conversion rates, for instance,” he says.

Not all retailers taking part on the overall Trusted Stores program have the program badges visible on their e-commerce sites. “We are still participating in the Google Trusted Stores Program, but we have taken the badge off while we’re in the processes of making it easier to use on our site,” said Overstock.com president Jonathan Johnson on Friday. “We should have the Google badge back on the site in the next week or so.”

A Google spokeswoman says some retailers in Trusted Stores might be part of the control group–that is, going without a badge in order to test the overall program’s impact.

April Anderson, industry director, retail, at Google, will speak at the Internet Retailer Conference & Exhibition 2012 in June, in a session entitled, “Paid search strategies for the smaller merchant.”