“Accordingly, training underscores a perpetual cycle: perform standardized work, encounter variation, conduct problem solving, set a new standard, train everyone to the new standard, and perform to the new standard. That sequence is the core of the daily activity for the team members, team leader, and group leader. Without this sequence there is never enough stability in the process to make improvements, and there is no system to standardize them and share them after they are made.” 

Toyota Culture  by Jeffrey Liker & Michael Hoseus (McGraw Hill, 2008) p. 139