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LAST UPDATED 20 SEPTEMBER 2016

Question 12: At what intervals should we measure the sample groups?

It is quite usual to adopt fixed measurement intervals, e.g. one sample group per hour. That, however, does not always work so well with SPC. We are dealing here with a live process, and the interval that is right during the time when a machine is warming up, for example, may not be the right one when it has reached equilibrium temperature. Another example is a tooling change, when initial wear may call for shorter measurement intervals.

If you measure at fixed intervals, there is a very great risk that the operator may feel forced to check single components between sample groups, and thereby misinterpret the result. He may wind up controlling the process on the basis of faulty data.

A good rule is to start by measuring at shorter intervals than you had envisaged. After a time the operator will find a suitable interval for measurement.

 

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12. At what intervals should we measure the sample groups? »

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