I read an article about a study of piano majors and their practice habits (one of those included in this survey)
. Students of all levels started sessions with approximately the same errors per minute - but the better players decreased those errors during the session. The ones who got better focused on correcting their errors, where the lesser players played to hear themselves play.
What do you tell your students? What works for you? How do you structure your own practice routine? Here's a link to my own post on the subject.