Google has started a validation period for a new system of calculating how online publishers are paid for displaying its referral ads.

Publishers of Web sites make money when visitors to their sites click the ads that Google places on their sites through its AdSense program. The ads that Google places on the publishers' sites are for products or services related to the content of the sites.

Google now wants to ensure that the conversions generated from referral ads are valid -- meaning that the people who click on the ads are actual customers -- according to Eva Woo, of AdSense product marketing, in an announcement on Google's AdSense blog. And that means that publishers who display the referral ads may not make as much money as they thought they would be making.

"The earnings you receive for the ad may now be less than the maximum referral value displayed for that ad," Woo said in the blog. "This is because our system will initially place a restriction on referral earnings as we monitor click and conversion data to determine that the conversions generated are valid. Once this validation period ends, you'll begin earning the maximum value of the conversions as displayed in your account."

Google did not say how long the validation period would last, but said it would vary from publisher to publisher. Google could not be reached for comment.

The company claims the change will have a minimal effect on most publishers.

But one publisher, Ken Dahlin, said that while he hasn't noticed a change in the money he receives from the program, he has noticed that he didn't make any money from a couple of conversions when he thought he should have been paid from them.

"I just figured that a single individual was taking advantage of the same deal," he said in a blog. "Whether that's fair or not, I didn't find it that big of a deal to start an argument. What troubles me is if I do get lower conversion prices during validation I'm going to have to lower my budget for promoting those links, which could kill my business model. In addition, the announcement says nothing about getting the full price after the conversion is deemed valid. We'll see."