I wouldn't say spaced repetition is un-studied, but it did sort of come in and out of vogue since its discovery. Look at the references here: https://en.wikiped

I wouldn't say spaced repetition is un-studied, but it did sort of come in and out of vogue since its discovery. Look at the references here: https://en.wikipedia.org/wiki/Spaced_repetition#References https://en.wikipedia.org/wiki/Spaced_repetition#References One of the studies even differentiates between long- and short- term retention, as impacted by the timing of the repetitions. Pimsleur, possibly coincidentally, follows the method they describe as more appropriate for long-term retention.