Wikipedia: Forgetting curve

The speed of forgetting depends on a number of factors such as the difficulty of the learned material (e.g. how meaningful it is), its representation (see: mnemonic techniques), physiological factors such as stress and sleep. Interestingly, the [basal forgetting rate]? differs little between individuals. The difference in performance (e.g. at school) can be explained by mnemonic representation skills. This means that some kids are able to "imagine" memories in the right way while others are not. Basic training in mnemonic techniques can help overcome those differences in part.

Forgetting curve is most steep for nonsensical material that will often even fail to make a mark in short-term memory. On the other hand, it is nearly flat for vivid or traumatic memories. The flattness of the curve is not an evidence of the decrease in the forgetting rate, but an evidence of implicit repetition (e.g. reliving memories) that indefinitely restores memory traces.

In a typical schoolbook application (e.g. learning word pairs), most of students show the retention of 90% after 3-6 days (depending on the material). This means that, in this period, the forgetting curve "falls" by 10%.

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