When asked why major marketers’ ad dollars don’t follow the vast migration of consumers to the web, the general answer is that online isn’t as powerful a branding medium as TV. A study of the internet’s ad effectiveness prepared by the Online Publishers Association aims to challenge that view somewhat. The study’s ambitions aren’t meant to dent traditional; instead it’s taking on the easier challenge of making the case for branded content sites versus portals and remnant ad networks.

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The study was based on data from researcher Dynamic Logic’s MarketNorms database, which covered 4 million respondents on 2,000 online ad campaigns over the past three years, through Q1 2008. The two main points OPA hopes to drive home is that major content sites, by dint of the familiarity and distinct identity they have with users, can improve brand favorability and purchase intent more than other sites. While not surprising that popular online outlets can transfer their bona fides to marketers’ messages, it does offer a glimpse of how the web might ultimately close the spending gap with traditional. In defense of the use of portals and remnant ad nets, marketers tend not to use them for branding purposes anyway; they tend to be more valued for direct response. So with that in mind, the full report is here (PDF) with some highlights below:

—Sponsorships on branded content sites were 42 percent more effective than the overall MarketNorms average and 36 percent more effective than on portals.

—18-34 year olds are more responsive to ads on branded content sites: they are 33 percent more likely to form positive opinions about advertised brands than when viewing ads on portals.

—Consumer packaged goods advertising on branded content sites gets a 26 percent lift in purchase intent over MarketNorms.