# File:First forgetting curve.jpg

Figure:The first forgetting curve (first repetition for items with no lapses). Unlike it was the case in earlier SuperMemos, where all forgetting curves were exponential, the first forgetting curve in SuperMemo 18 is approximated using power regression. This provides for a more accurate mapping due to the heterogeneity of the learning material introduced in the learning process that results in superposition of exponential forgetting with different decay constants. The use of power regression explains why the first interval might be slightly shorter in Algorithm SM-17. On a semi-log graph, the power regression curve is logarithmic (in yellow), and appearing almost straight. The curve shows that in the presented collection recall drops merely to 58% in four years, which can be explained by a high reuse of the memorized knowledge in real life. In earlier SuperMemos, recall data would only be collected in the span of 20 days, and negatively exponential forgetting curve would make for far lower retrievability predictions. The first optimum interval for the forgetting index of 10% is 3.76 days. The forgetting curve can be described with the formula R=0.987*power(interval,-0.07), where 0.987 is the recall on Day 1, while -0.07 is the decay constant. This is case, the formula yields 89.5% recall after 4 days, which is then used as the first rounded optimum interval. Almost 77,000 repetition cases were used to plot the presented graph. Steeper drop in recall will occur in collections with a higher mix of difficult items, in poorly formulated collections, or in new users with lesser mnemonic skills.

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