Control Limits
Every process has natural, everyday ups and downs. No two coffee cups are filled to the exact same drop; no two phone calls take precisely the same number of seconds. "Control limits" describe the normal range you should expect that natural variation to stay within, as long as nothing has gone wrong.
The formula is CL = mean ± 3σ. Remember, the "mean" is your average result, and σ (sigma) is the standard deviation — a measure of how much your results normally spread out. You take the average, then set an Upper Control Limit three standard deviations above it and a Lower Control Limit three standard deviations below it. Together these two lines mark the boundaries of "normal."
Here's how to read it: if your measurements stay between the upper and lower control limits, your process is behaving predictably and you leave it alone. If a point jumps outside the limits, that's a warning sign that something unusual happened and you should investigate. Project managers use control limits on "control charts" to keep an eye on quality over time and catch problems early — before they grow.
Think of it like the guardrails on a highway. A little bit of drifting left and right within the lane is completely normal driving. But if the car crosses the guardrail, something has clearly gone wrong and needs attention right away.
Suppose a bakery's bread loaves weigh 500 grams on average, with a standard deviation of 10 grams. The Upper Control Limit is 500 + (3 × 10) = 530 grams, and the Lower Control Limit is 500 − (3 × 10) = 470 grams. So any loaf between 470g and 530g is perfectly normal. But if a loaf comes out at 545g, it's outside the limits — a signal that the machine or recipe may need checking.
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