Learning Versus Performance: An Integrative Review, Nicholas C. Soderstrom and Robert A. Bjork, Perspectives on Psychological Science 2015, Vol. 10(2) 176–199

What we see in practice is an unreliable index of whats going on in terms of learning. We can only observe the indirect performance effect of learning, but the way learning is measured is by using a retention test.

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Evidence from both the motor- and verbal-learning domains shows that retrieval practice can have opposing effects on learning and performance. Motor-learning studies have revealed that, on the whole, physical guidance often reduces performance errors during training but that unguided, active involvement promotes better long-term retention of skills. Likewise, practicing retrieval of verbal materials may appear unhelpful during acquisition and on immediate memory tests, but it provides substantial benefits in preserving or stabilizing long-term memory. It would seem prudent, therefore, that trainers and instructors incorporate retrieval practice into their curriculum and that students test themselves as a means to optimize their own learning