A method or means of providing instant feedback when the learner is engaged in using an educational material or device so the learner can correct his errors independently and allows the learner to use his reason, critical thinking capacity and his increasing capacity for drawing distinctions. (Boehnlein)

“So we come to a scientific principle which is also a path to perfection. We call it ‘the control of error.’ Whatever is done in school, by teachers, children, or others, there are bound to be mistakes. So we need this rule as part of school life, namely, that what matters is not so much correction in itself as that each individual should become aware of his own errors. Each should have a means of checking, so that he can tell if he is right or not.” (Montessori, Maria, The Absorbent Mind, p. 224)