Control limits, also known as natural process limits, are horizontal lines drawn on a statistical process control chart, usually at a distance of ±3 standard deviations of the plotted statistic from the statistic's mean.
Control limits should not be confused with tolerance limits or specifications, which are completely independent of the distribution of the plotted sample statistic. Control limits describe what a process is capable of producing (sometimes referred to as the “voice of the process”), while tolerances and specifications describe how the product should perform to meet the customer's expectations (referred to as the “voice of the customer”).
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Famous quotes containing the words control and/or limits:
“Imagine believing in the control of inflation by curbing the money supply! That is like deciding to stop your dog fouling the sidewalk by plugging up its rear end. It is highly unlikely to succeed, but if it does it kills the hound.”
—Michael D. Stephens. “On Sinai, There’s No Economics,” New York Times (Nov. 13, 1981)
“As nature requires whirlwinds and cyclones to release its excessive force in a violent revolt against its own existence, so the spirit requires a demonic human being from time to time whose excessive strength rebels against the community of thought and the monotony of morality ... only by looking at those beyond its limits does humanity come to know its own utmost limits.”
—Stefan Zweig (18811942)