Internet companies have had great success selling advertising space, in part because the effectiveness of those ads is supposedly so easily measured. But marketers, even as they continue to push more of their ad budgets online, are starting to ask for better proof.

A group of large companies, including Kimberly-Clark, Colgate-Palmolive and Ford Motor have said that by the middle of 2007, they will demand that online publishers hire auditors to check their ad and viewer counts. And analysts say they believe that online ad growth over the long haul will depend on the eagerness of large advertisers like these to shift more dollars online.

Meanwhile, reacting to advertiser questions, online companies like Google, Yahoo and LookSmart have begun to meet with industry groups to answer basic questions on how click-based advertising works.

Other companies are concerned that so-called click fraud may be driving up their ad bills, so they are sharing their proprietary ad data with click-trackers, which try to figure out how prevalent such devious clicks are.

Get the full story at The New York Times