Google is at it again. On June 24, the search giant unveiled its new Ad Planner service via a post on its AdWords Blog. Its latest foray into the analytics category, Ad Planner is currently in pilot form, being offered by invitation only and free for a limited time. Aimed specifically at media planners, the new service is coming directly off the heels of last week's introduction of Google Web Trends—though Web Trends is targeted toward all Internet users.

Ad Planner allows media professionals to find the Web sites an advertiser's target audience is likely to visit based on metrics such as demographics and site traffic. Ad Planner users can drill down using more specific filters such as age, gender, education and household income. The service can also give planners and advertisers information on a site's international reach and unique visitors, as well as a list of keywords that visitors used in searches.

Google, already the leader in search, is now looking to strengthen its analytics services in an effort to capture more display advertising revenues. Ad Planner will make highly coveted Web tracking data available to advertising professionals for free, essentially opening up the ivory tower to smaller advertisers and agencies. Some industry experts believe it could be a boon for small agencies, as media planners at these firms do not always have the budget for more expensive Web tracking resources.

"Google's deep pockets allow the company to create free offerings, such as Ad Planner, as a useful come-on to marketers to gain their ad business," said David Hallerman, senior analyst at eMarketer. "However, continued uncertainty about which company's Web measurements are most accurate could get exacerbated—or clarified—by another analytics service."

The introduction of Ad Planner appears to position Google as a direct competitor to firms such as comScore, Nielsen Online and Hitwise. Though each service analyzes sites based on similar metrics, such as traffic and demographics, their methodologies and business models differ. comScore, Nielsen and Hitwise charge subscription fees to access their data, while Ad Planner currently does not.

comScore and Nielsen also use panels and surveys of volunteer Web users to collect data. Hitwise takes a network-centric approach, collecting its usage data directly from ISP networks. Ad Planner data is gathered, in part, via the Google Toolbar. Google can also leverage data from other sources, such as its search results, third-party networks and other partner market research firms—though the company has not released a list of exactly which sources it will be including.

Each service's methodology, however, has drawn criticism from industry experts. Panel and survey search strategies have come under fire in the past, the main argument being that Web usage is too fragmented to be accurately represented by a limited sample size. Reporting solid results about Internet usage becomes tricky when using a restricted number of participants. Smaller sites may not be included in results, and the selected panel may not accurately represent the online population as a whole.

Meanwhile, Google leads in search, with a market share of nearly 70%. This means that more than 30% of searches are not being tracked by the company and will not be included in Ad Planner data.

Some experts wonder if Google's Ad Planner service may be a conflict of interest. Will advertisers trust data from a source that sells them online advertising?

"Since Google's purchase of DoubleClick, the search giant—even more than before—needs to walk a fine line," said Mr. Hallerman. "While publishers use their own traffic data all the time to sell ads on their pages, the Ad Planner service can also be used more objectively, with data from sites Google has no control over or interest in."

Most pundits predict that comScore and Nielsen will weather the Ad Planner storm. These services currently offer more detailed results than Ad Planner, as well as the integration of offline information and the ability to cut data in more ways. comScore and Nielsen Online may focus on their premium services as a distinguishing characteristic, while touting their custom research capabilities. Some analysts even propose that Google's new service may be better compared to other free Web trackers, such as Amazon.com's Alexa, Compete.com and Experian.

Ad Planner appears to be the next step in Google's evolution of advertising services. Mr. Hallerman notes that as Ad Planner develops it will likely have two levels, like many other online services: a free service for basic offerings and a subscription level for additional functionality.

"Keep in mind that Ad Planner is only in beta right now," warned Mr. Hallerman. "Perhaps, like other Google offerings that are released in beta, it might take some time before getting fully developed. However, this type of Web analytics will improve as more companies use it, generating more data for ad measurement."