The spacing effect demonstrates that learning is more effective when study sessions are spaced out. This effect shows that more information is encoded into long-term memory by spaced study sessions, also known as spaced repetition or spaced presentation, than by massed presentation ("cramming"). The phenomenon was first identified by Hermann Ebbinghaus, and his detailed study of it was published in the 1885 book (Memory: A Contribution to Experimental Psychology), which suggests that active recall with increasing time intervals reduces the probability of forgetting information. This robust finding has been supported by studies of many explicit memory tasks such as free recall, recognition, cued-recall, and frequency estimation (for reviews see Crowder 1976; Greene, 1989). Researchers have offered several possible explanations of the spacing effect, and much research has been conducted that supports its impact on recall. In spite of these findings, the robustness of this phenomenon and its resistance to experimental manipulation have made empirical testing of its parameters difficult. Causes Decades of research on memory and recall have produced many different theories and findings on the spacing effect. In a study conducted by Cepeda et al. (2006) participants who used spaced practice on memory tasks outperformed those using massed practice in 259 out of 271 cases. As different studies support different aspects of this effect, some now believe that an appropriate account should be multi-factorial, and at present, different mechanisms are invoked to account for the spacing effect in free recall and in cued-memory tasks. Not much attention has been given to the study of the spacing effect in long-term retention tests. Shaughnessy (1977) found that the spacing effect is not robust for items presented twice after a 24-hour delay in testing. The spacing effect is present, however, for items presented four or six times and tested after a 24-hour delay. The result was interesting because other studies using only twice-presented items have shown a strong spacing effect, although the lag between learning and testing was longer. Shaughnessy interprets it as evidence that no single explanatory mechanism can be used to account for the various manifestations of the spacing effect. Semantic priming Research has shown reliable spacing effects in cued-memory tasks under incidental learning conditions, where semantic analysis is encouraged through orienting tasks (Challis, 1993; Russo & Mammaralla, 2002). Challis found a spacing effect for target words using a frequency estimation task after words were incidentally analyzed semantically. However, no