There are a lot of ad networks out there, and each one claims to have the reach marketers are looking for. But now, comScore will put some of those claims under the spotlight when it rolls out a new reporting structure for ad networks next month.

The new reporting regime will provide two metrics for marketers: "potential reach" and "actual reach."

Potential reach will measure the unduplicated visitors to each site in the network by looking at written documentation -- provided by comScore and confirmed by the network -- to determine how many people could possibly visit a given set of sites. By contrast, actual reach will measure the real number of ads served on the site during a specific reporting period. To determine actual reach, comScore will require networks to abide by their reporting protocol, which means that not all networks will be part of the new reporting regime.

"We believe these two toolsets provide the information that ad agencies need when they are deciding on which advertising networks to consider for ad buys. And, the ad networks will have the high quality, third-party information needed to compete for advertising dollars," said Magid Abraham, president and CEO of comScore. "Ad networks are a vital means of monetizing most websites and delivering high reach to advertisers. These new metrics provides ad networks, agencies and advertisers with the accurate information needed for buying and selling advertising."