Control Chart Question.

Submitted by anmol2701 on Thu, 04/24/2014 - 09:11
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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?
 
  The process is under control. It should not be adjusted.
  A special cause should be investigated, the process should be adjusted.
  The control limits should be adjusted.
  The measuring equipment should be recalibrated.

 

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anmol2701

Thu, 04/24/2014 - 09:14

My answer is option 1. As all the limits are under control...

But the answer is option 2 according to http://www.oliverlehmann.com/pmp-self-test/75-free-questions.htm#testbegin

Can anybody point me what I missed.

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anmol2701

Thu, 04/24/2014 - 09:18

http://pmzilla.com/here-question-control-limits-i-am-very-confused-answer-pmp

 

Got answer. :-)

sunku65

Thu, 04/24/2014 - 13:48

Answer is B.

Explanation:

The data points ...19.970cm, 19.968cm, 19.963cm, 19.958cm, 19.962cm, 19.979cm, 19.959cm are below one side of mean i.e. seven data points called "Rule of Seven".  Its a Assignable clause(special cause) need to be investigated and the process is out of contorl. need to be adjusted.

 

 

philstinnet

Thu, 04/24/2014 - 18:05

Please remember that PMBOK or RITA or any other book on PMP exam preparation is not a book on quality. Therefore, they do not have the good information on Quality.

There are many rules which will require one to investigate the process -

The rule of Seven consecutive data points on one side of the mean - This is really not a rule. It is just a company's quality metrics. Some companies say that it should be 8 and other use 9 data points to call it - an assignable cause has occured.

Reason - A probability of a data point on one side of the mean line = 0.5 = 50%.

The probability of the next data point on the same side of the mean line = 0.25 = 25%.  

The probability of the 3rd data point on the same side of the mean line = 0.125 = 12.5%

The probability of the 4th data point on the same side of the mean line = 0.0625 = 6.25%

The probability of the 5th data point on the same side of the mean line = 0.031 = 3.1%

The probability of the 6th data point on the same side of the mean line = 0.016 = 1.6%

The probability of the 7th data point on the same side of the mean line = 0.008 = 0.8%

The probability of the 8th data point on the same side of the mean line = 0.004 = 0.4%

The probability of the 9th data point on the same side of the mean line = 0.002 = 0.2%

And now, the probability of a data point outside the UCL and LCL = 0.27%

Now, you can clearly see that why some people choose and call it the 'rule of 9' and some call it 'the rule of 8'. But, PMBOK calls it 'rule of 7'.

Whatever floats the boat.... 

sunku65

Fri, 04/25/2014 - 04:53

we need to follow PMBOK. The question din't say any overriding PMBOK  policies.  as of now according to PMBOK its a Rule of seven.  The answer is B.   The companies creates their own quality policy, under this policy they can have no.of metric rules like  6, 8, 9 and 10. That is not the matter here.  The matter is only PMBOK. 

 

philstinnet

Fri, 04/25/2014 - 15:05

Indeed.... that is why i ended my sentence with - "Whatever floats the boat.... "