It has long been known that spacing practice (reviewing learning or practicing a skill at spaced intervals) is far more effective than massed practice (in one heavy session). It is also well-known that people commonly over-estimate the value of massed practice, and tend not to give due recognition to the value of spacing practice, despite the fact that most memory improvement and study programs advise it.
Many learning strategies require extensive training. The advantage of spaced practice is that it does not. Experience with it may also result in better self-appraisal about how well information has been learned.
In this study, a class of 708 students were given instruction sheets on which was written a 7-digit number purporting to be a phone number. The students were instructed to memorize the number and told their recall would be tested later in the term. Half the class were told to memorize the number however they usually would. The other half were told to post the number where they would see it, and look at it once or twice a day for a week. They were told this would be an effective way of learning the number.
A significantly greater number of students from the spaced-practice group remembered the number correctly two weeks later (72.7% compared to 61% of the control group). According to the questions they answered, some 11.6% of the spaced-practice group in fact did all their studying in a single session, and only 46.4% studied the number on 3 or more days. Some 18% of the control group also studied the number on 3 or more days. In other words, being in the spaced-practice group doesn't necessarily mean spaced practice was used, nor does being a member of the control group mean that spaced-practice wasn't used.
Clearly, simple instructions to use spaced practice improve memory, but equally clearly, many people are not necessarily going to follow those instructions, for whatever reason.