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.
Faq´s about SPC
1. Why should we use SPC in my company? »
2. Where should we start using SPC in our operations? »
3. What can we achieve with SPC? »
4. Can we start to introduce SPC without any preparation? »
5. What is MSA? »
6. What should we think about before starting to use SPC? »
7. What software should we use? »
8. What is the difference between producing with and without SPC? »
9. Where should the control limits be located on the control chart? »
10. What capability index should we have as a minimum requirement? »
11. We produce short runs. Is SPC any use to us? »
12. At what intervals should we measure the sample groups? »