Since its debut in January, the USC Annenberg Innovation Lab Ad Transparency Report has informed readers of various brands’ decisions about online advertisement spending.

The report is a monthly public memo that calls attention to the online ad networks and consumer brands that — intentionally or not — serve as illicit piracy websites.

Variety.com first published an article which showed that Google and Yahoo! were among the top ad networks with piracy-site ad placement.

Google and Yahoo! have now responded to the publication of the Transparency Report by reaching out to the Annenberg Innovation Lab and working to eliminate its connections to content theft. Ad agencies, such as Group M and Omnicom, have likewise worked to prevent their campaigns from showing up on piracy sites.

“The report has brought some sunlight to an issue that people weren’t concentrating on,” said Jonathan Taplin, director of the Annenberg Lab and professor at the USC Annenberg School for Communication and Journalism. “It’s like any good research report — you look at something that was under a rock and you lift up the rock and see what’s underneath. And it wasn’t pretty.”

Major media outlets, such as The New York Times, Guardian and Los Angeles Times, have covered the Transparency Report, as have a slew of niche publications that focus on the advertising, entertainment and file-sharing industries.

“The ad business was the least transparent business that I know,” Taplin said. “People don’t really understand how it works. They don’t know who the players are. They don’t know how those ads end up on your computer or why it seems like brands are following you around wherever you go.”

Previously, that confusion contributed to the difficulty in determining how advertisements from both prominent and less prominent companies made their way to piracy sites.

Occasional glimpses into that domain demonstrated that the connection could be lucrative. A report by Google and PRS for Music determined that 86 percent of peer-to-peer piracy sites were advertiser-supported. One court case even revealed how one notorious site was offered $100,000 per month to post gambling ads.

“It’s really an issue of, ‘Where is the money in this business?’ ” Taplin said. “And if the money that paid for all of [entrepreneur] Kim Dotcom’s yachts and his four mansions and everything came from advertising, you don’t need a law to stop that. You just need the people in the business to pay some attention and develop a series of best practices.”

Each Ad Transparency Report has specified the top 10 ad networks that bring the largest number of ads to piracy sites. The January report placed Openx and Google in the top two slots as the greatest offenders. That inaugural report was based on a year’s worth of data gleaned from the Google Transparency Report.

By the time the Transparency Report was published in February, both Openx and Google had dropped out of the top 10.

“We’ve seen radical changes in Google’s behavior,” Taplin said. “We’re beginning to see radical changes in Yahoo!’s behavior. And Quantcast, another fairly big player, has changed their behavior quite rapidly.”

Monthly editions of the USC Annenberg Innovation Lab Ad Transparency Report are posted here.

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