If we consider any process, it could be the machining of a shaft
diameter or the time taken to answer the phone in a call centre, we know
it will vary. If we collect data over time there will be an average
diameter or answering time and the individual values will be dispersed
around the average.
f we know nothing about statistics and variation, then we may try to
adjust the machine or do something about call answering times each time
we see a value either above or below the average. But what about if the
diameter is too low and we adjust it upward but the next value would
have been higher anyway? Adjusting the process in this way just makes
performance worse, it is technically known as over adjustment.