When Should We Compute New Limits?

How to use the limits to track the process

Mon, 04/02/2012 - 14:51

Last month in “Exact Answers to the Wrong Questions” we looked at how we can compute useful limits with as few as six to 10 values. In this column I would like to consider the question of how to use the limits on a process behavior chart to understand the underlying process. In order to do this, we need to answer the related question, “When should we compute new limits?”

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This question about computing new limits usually takes one of two forms. Most often it has to do with “When do we need to update the limits of a continuing chart?” Occasionally it has to do with “When do we need to polish the limits that we have just computed?” I will discuss both of these issues in this column.

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Comments

Submitted by kkbari on Mon, 04/09/2012 - 11:09

I like the 4-step questions - good to use for those not intimately familiar with process control but in charge of a process.  Will put that in my bag of tools!

However, in my years of being asked this question, the source of this question is not a matter of the process but stems from a chicken and egg scenario.  Supplier A defined original limits (let's even imagine they were correct).  Process changed over time (better or worse), reasons both known and unknown.  People move on.  No one pays attention.  Customer comes in for audit.  Sees charts are now industrial wallpaper and suggests/insists upon timely review. 

Since supplier A didn't know how to manage process control, Customer makes all suppliers review limits against the calendar.  Supplier B tries to explain the exact points you make but is shot down quickly.  If I'm the customer I want all of my suppliers to be paying attention to reduce my incoming variability. 

Years ago, I put in our company policy that there is no timeframe for recalculating limits and put a short statement similar to your premise. 

Good article for many out there fighting Purchasing departments.

Submitted by kkbari on Mon, 04/09/2012 - 11:10

I like the 4-step questions - good to use for those not intimately familiar with process control but in charge of a process.  Will put that in my bag of tools!

However, in my years of being asked this question, the source of this question is not a matter of the process but stems from a chicken and egg scenario.  Supplier A defined original limits (let's even imagine they were correct).  Process changed over time (better or worse), reasons both known and unknown.  People move on.  No one pays attention.  Customer comes in for audit.  Sees charts are now industrial wallpaper and suggests/insists upon timely review. 

Since supplier A didn't know how to manage process control, Customer makes all suppliers review limits against the calendar.  Supplier B tries to explain the exact points you make but is shot down quickly.  If I'm the customer I want all of my suppliers to be paying attention to reduce my incoming variability. 

Years ago, I put in our company policy that there is no timeframe for recalculating limits and put a short statement similar to your premise. 

Good article for many out there fighting Purchasing departments.

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