enhance memory in an account known as the retrieval effort hypothesis. Spacing out the learning and relearning of items leads to a more effortful retrieval which provides for deeper processing of the item. Practical applications and long-term retention Advertising The spacing effect and its underlying mechanisms have important applications to the world of advertising. For instance, the spacing effect dictates that it is not an effective advertising strategy to present the same commercial back-to-back (massed repetition). Spaced ads were remembered better than ads that had been repeated back to back. Layout variations presented in short spacing intervals also resulted in improved recall compared to ads presented in exact repetition. The same effect was also achieved in a study involving website advertisements. It was revealed that sales diminish progressively as the customer visited the site and was exposed to the ad several times. However, if the elapsed time between the visits was longer, the advertisement had a bigger effect on sales. If encoding variability is an important mechanism of the spacing effect, then a good advertising strategy might include a distributed presentation of different versions of the same ad. Appleton-Knapp, Bjork and Wickens (2005) examined the effects of spacing on advertising. They found that spaced repetitions of advertisements are more affected by study-phase retrieval processes than encoding variability. They also found that at long intervals, varying the presentation of a given ad is not effective in producing higher recall rates among subjects (as predicted by variable encoding). Despite this finding, recognition is not affected by variations in an ad at long intervals. Application in education Studies have shown that long-term spacing effects are prevalent in learning and produce significant learning gains, particularly when the spacing gaps are on the order of days or weeks. Although it is accepted that spacing is beneficial in learning a subject well and previous units should be revisited and practiced, textbooks are written in discrete chapters that do not support these findings. Rohrer conducted a two-part study in 2006 where students were taught how to solve math problems. In part 1, students either used mass or spaced practice, and spaced practice showed significant improvement over mass practice when tested one week later. In the second part of the experiment, practice problems were either grouped by type or mixed randomly. The desirable difficulties encountered by the randomly mixed problems were effective, and the performance by students who