Lessons · Project management · a control chart
A control chart: is the process wobbling normally, or has something changed
A control chart plots a measurement over time against its mean and control limits at plus and minus three standard deviations; a point outside the limits, or seven in a row on one side of the mean, says the process has changed.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
The daily defect count goes from 4 to 6. Is that a problem? If the mean is 5 and the limit is 9, it is Tuesday. If seven days in a row have been above 5, something has changed even though no single day looks bad. The chart is what stops you reacting to noise and ignoring signal.
How to think about it
Compute the mean and the standard deviation from a stable period. Draw the mean, the upper limit at mean + 3 SD and the lower at mean − 3 SD. Plot every new point. Act on a point outside the limits or a run of seven on one side; leave the rest alone.
Worked example
Mean 50.0 mm, SD 0.2 mmFrom twenty parts measured on a good day.
UCL = 50.0 + 3 × 0.2 = 50.6 mm. LCL = 50.0 − 3 × 0.2 = 49.4 mmThe limits.
Today's part: 50.5 mmInside the limits. Normal wobble; do nothing.
Seven parts in a row between 50.2 and 50.5 mmAll inside, all above the mean. The rule of seven: the process has drifted. Investigate.
Your turn
Mean 20 and SD 1.5. Write the upper control limit.
UCL = 20 + 3 × = 24.5
Solve one, graded on the server
The trap
Adjusting the process after every point above the mean. Half of all points are above the mean. Chasing them adds variation instead of removing it.