Peter Steen Høgenhaug on the results of his icon usability tests:

All in all, only 35.29 % of the test participants understood the chain icon, and only 25 % understood the globe icon. While we didn’t set a minimum for success, clearly, 25 % is not good when it comes to usability.

While these tests provide some good insight, they completely miss the point that icons were never meant to replace text labels to start with.

Thanks to HackerNews for the reminder.