Any user with 15,000 or more reputation can protect a question, making it so that users with less than 10 reputation cannot answer it. This is usually done to prevent spam, non-answers, very low-quality answers, and so forth. Of course, it’s impossible for the average user to see how effective this is, since they can’t see how many answers a <10 rep user has not posted. It’s often quite difficult to see whether protecting a question actually made anything better, or whether the question was just returning to its average level of bad answers.
Is there any measure of the effectiveness of protecting questions, in terms of how many low-reputation users have been prevented from posting, for example?