 However, when diary data is collected on paper, it is not known when the ratings are actually made, and there is evidence that compliance may be quite poor.In one study, patients were given an instrumented paper diary that recorded covertly when it was opened.The study showed frequent cases of "back-filling", filling in a batch of entries some time, often days, after they were due, and even in some cases of "forward filling", completing entries before they were due.