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Topic History of : Special Cause Variation- Control Charts

Max. showing the last 6 posts - (Last post first)
7 years 3 months ago #9123

Henry ILO

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Thanks for the feedback Rahul. Apologies for posting in wrong section but Please concerning ITTOs and process flow for the 47 processes, please whats your advice? Your kind revert is most appreciated. Thanks. This email address is being protected from spambots. You need JavaScript enabled to view it.
7 years 3 months ago #9096

KBG

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Rahul-
That's great!
Thank you for your clarifications.
It sure did help me!
7 years 3 months ago #9091

Rahul Kakkar

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Hi Kristin,

Yes, that's exactly correct. And yes you can expect such question but the only calculation that is required is to take the sum and divide by the number of measurements (mean or average).

After that's its an eyeball check to see how many are below and above the mean.

Hope this helps.
7 years 3 months ago #9088

KBG

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Hi Rahul,
Based on these values, can you clarify this is how you calculated to get the solution.

20.033cm, 19.982cm, 19,995cm, 20.006cm, 19.968cm, 19.963cm, 19.958cm, 19.979cm, 19.959cm.
mean/ average= 19.983 (179.843/9) add all # values/divide by sum of #'s
below the mean: 19.982, 19.963, 19.958, 19.979, 19.959
above the mean: 20.033, 19.995, 20.066

Overall, more than 5 consecutive points on one side of the average value (below the mean) Therefore, a special cause could
be investigated, and the process should be adjusted.
Thank you!
Kristin
7 years 3 months ago #9087

KBG

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Hi Rahul,

Thank you for your detailed response, very helpful!
Will we have to make these calculations on the exam or will it be more of understanding what common causes vs special causes mean?
I'm asking b/c it can be time consuming.

Thanks.
7 years 3 months ago #9085

Rahul Kakkar

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Hi Kristin,

Let's first discuss the difference between common cause and special causes.
On a control chart, common cause variations would have the pattern of
  • all points within the control limits
  • points distributed randomly on both sides of the average value
On a control chart, special cause variations would have the pattern of either:
  • a point or more beyond the control limits
  • some trends of the points (e.g. more than 5 consecutive points on one side of the average value)
Now, taking the numbers given in the question, the mean of the measurements comes to 19.983. If you see, 6 values are below the mean and 3 values are above the mean. Therefore, this hits the condition - "some trends of the points (e.g. more than 5 consecutive points on one side of the average value)" and as a result it is a special cause which could be investigaed and the process should be adjusted.

Hope this helps.

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