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Periodic reinforcement/repetition after training greatly helps retention. This is a very effective way of making training stick i.e. create a permanent impression on the trainees. The above graph shows my attempt at capturing the effect of periodic reinforcement (post-training).
Without periodic reinforcement, the knowledge/skill level attained through training starts dropping off due to the effect of time. With periodic repetition/application, it is possible to keep the average skill level from decaying. After some time, when the learning has been imbibed fully and becomes second nature, the reinforcement may be discontinued.