When To Calculate, Lock and Recalculate Control Limits

Find the answer to these questions:
• When do I calculate control limits?
• When do I “lock” control limits in place?
• When do I recalculate control limits?
• How many points do I need before I can calculate a control limits?
• How many points have to be “in-control” before I have “good” control limits?
• How do out of control points impact the control limit calculations?
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[@kopytko998]

Very good video. Thank you. I still have one question. How do you calculate limits for the first 5 points? For 20 points you can use 3 sigma values, but for 5 points you need to correct it - I think (number of points is too low).

[@SPCforExcel]

The control limits are based on the average range from a range chart, for example a moving range of 2 for the individulas chart. Please see our SPC Knowledge Base for more information: www.spcforexcel.com/spc-for-excel-publications-category

[@RNE365]

Many years ago, Perry Regier formulated a set of four questions to use in determining if you need to revise the limits. 1. Do the data display a distinctly different kind of behavior than in the past? 2. Do you know the reason for this change in behavior? 3. Is the new process behavior desirable? 4. Is it intended and expected that the new behavior will continue? If the answer to all four questions is yes, then it is appropriate to revise the limits based on data collected since the change in the process. If the answer to question 1 is no, then there should be no need for new limits. If the answer to question 2 is no, then you should be looking for the Assignable Cause instead of tinkering with the limits. If the answer to question 3 is no, then why are you not working to remove the detrimental Assignable Cause instead of tinkering with the limits? If the answer to question 4 is no, then you should again be looking for the Assignable Cause instead of tinkering with the limits.

[@SPCforExcel]

Thanks for the comment. I like the four questions - coverd in the video, but not in that succinct order. Thanks for sharing this!