Third Principle of SPC – Things Vary in a pattern

The Third Principle of SPC is extension of the second one. In the last post on Second Principle of SPC,  I mentioned that we notice a Feature if the measurements of the output are analysed.

If we want to see the pattern, all we need to do is plot the individual data points or the measures taken onto a tally form. we will see definite pattern begin to form after several measurements are plotted.
An easy way to demonstrate this is to roll a pair of dice about 50 times or more and record them on a tally sheet. the pattern we see is a frequency distribution.
We can make a frequency distribution curve by enclosing the tally marks in a curved line. The curve you see will have more measurements at the middle and fewer as we go away from middle. It can be seen that the curve looks like a bell.
Whenver one takes a group of measurements, a frequency distribution curve appears.

This would be explaind by the fourth basic principle of Statistical Process Control (SPC)

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