“As [Misaki] Imai explained so well in [his book] Kaizen, it is impossible to improve any process until it is standardized. If the process is shifting from here to there, then any improvement will just be one more variation that will be altered by the next variation. One must standardize, and thus stabilize, the process before continuous improvements can be made.” 

The Toyota Way 2nd Edition by Jeffrey K. Liker (McGraw Hill, 2021) p. 112.