Internal changeover represents the elapsed time between the production/processing of the last piece (part, transaction, or service) of the prior run and the first good piece of the next run. External changeover represents the elapsed time during which associates conduct the preparatory activities for the changeover BEFORE the last piece of the prior run and/or the elapsed time for any changeover related activities AFTER the first good piece of the next run. Changeover (a.k.a. "setup," see here for further insight) reduction strategy is summarized in the nearby figure.
Related Posts: Available Time for Changeovers, Changeover Time, Every Part Every Interval (EPEI)