One of the features app makers and online advertisers rely on is reliable measurement of their audiences and customers. But according to Scout Analytics, a Seattle-area firm that provides behavioral analytics tools for online publishers, audiences may be overcounted as much as 25 percent because of Web surfers who delete Flash-related cookies from their browsers and Flash players, either manually or via tools such as Flash Cookie Cleaner.

“Beyond the fact that unique user counts are overstated 2-4X by tracking devices,” Scout senior vice president of strategy Matt Shanahan emailed me after a phone interview, “cookies add additional errors in user counts. In our recent study, deletion rates have approached 7% for Flash cookies and more than 30% for HTTP cookies. These deletion rates further skew unique user count.”

A 7 percent deletion rate can cause up to a 25 percent overcount in audience size, Shanahan told me.

Moreover, he said, Adobe’s upcoming Flash 10.1 player will have a private browsing feature that blocks cookies from being stored on the user’s computer as part of a Flash session. Analysts will probably need to work in a fudge factor to account for uncookied users.