Millions of computer users collectively transcribe the equivalent of 160 books each day with better than 99 percent accuracy, despite the fact that few spend more than a few seconds on the task and that most do not realize they are doing valuable work, Carnegie Mellon University researchers reported today in Science Express.They can work so prodigiously because Carnegie Mellon computer scientists led by Luis von Ahn have taken a widely used Web site security measure, called a CAPTCHA.