Here is a question on Control Limits I am very confused with the answer

A production process has been defined as part of an industrial equipment manufacturing project. The process is intended to produce steel bolts with a length of 20 cm. The control limits are 19.955cm and 20.045cm.
The measurements made at the end of the process yielded the following results:

20.033cm, 19.982cm, 19,995cm, 20.006cm, 19.970cm, 19.968cm, 19.963cm, 19.958cm, 19.962cm, 19.979cm, 19.959cm.

What should be done?
 
 
A.   The process is under control. It should not be adjusted.
B.   A special cause should be investigated, the process should be adjusted.
c.   The control limits should be adjusted.
D.   The measuring equipment should be recalibrated.

 


I picked the answer A., Reason if 7 consecutive times if it goes out of limit, then it should be stopped and investigated.
But here it did'nt go 7 times out. so i chose A.

 

the answer chosen is B.(PMBOK® Guide 4th Edition, pp 196, 209)

Here is the link the question no: 3

http://www.oliverlehmann.com/pmp-self-test/75-free-questions.htm

 

 


 

Hi cplusplus,

Rule of 7 didnt say that when 7 consecutive times if it goes out of limit, then it should be stopped and investigated. it says that - If there are seven points on one side of mean, then an assignable cause must be found.

Hope it will clarify.

A process is considered not stable

a) When it is out of control limit

or b) Seven points are on either side of the mean

In PMBOK Guide 4 page no 227 under top control charts its mentioned as process is out of control if seven consecutive points are above of below the mean.

 


 

If you observe carefully, there is pattern. There are peaks uniformly placed which helps to identify specific reason which is controlling this behaviour of o/p. Hence it needs correction. And: B

"A signal is defined as any single point outside of the control limits. A process is also considered out of control if there are seven consecutive points, still inside the control limits but on one single side of the mean." from Wiki

So the value in middle is 20, last 7 of values there are under 20 so this means 7 consecutive points on one side. So there you go you have fail, so you should check whats up.

I almost fell for option A as well; CONSECUTIVE points on either side of the mean (20.0) implies only the last 7points hav violated.

Thanks stanislav for ur explanations.

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