Research on why repetition (with variation) increases probability of retention
There are many excellent papers with research supporting why repetition with variation has a high probability of increasing retention for children.
This research supports the view at KeyStageMaths.com that repetition with variation is an essential and often overlooked method to help children become fluent in numeracy.
Citations on education using repetition with variation
Here are a few citations:
Bruner (1961) stated that it is only through the exercise of problem solving and the effort of discovery that one learns the working heuristics of discovery. The more one has practice, the more likely is one to generalize what one has learned into a style of problem solving or inquiry that serves for any kind of task or almost any kind of task.
Marton, Wen and Wong (2005) pointed out that the likelihood of being able to recall something is higher if the learners hear or see something several times than if they do not. Furthermore, they commented that, unlike when you read the same presentation of something several times in the same way and thus repeat the same thing again and again, or read the same presentation in different ways, something is repeated and something is varied.
Noche & Yu (2015) found out from her study on supplemental self-paced instruction that focuses on the mastery of either concepts or procedures through repetition with variation, helps young adults improve their performance in tasks designed and proportional reasoning understanding and skills.