control chart used in the Perform Quality Control process Question

Submitted by cplusplus on Wed, 02/10/2010 - 15:55

In a control chart used in the Perform Quality Control process, which of the following statements is LEAST true?


A.  The upper customer specification limit represents the project's quality metric for the work performance information.
B.  A data observation below the lower control limit yet above the lower customer specification limit represents a process that is out of control and a product that is within specification.
C.  A data observation that is above the lower control limit and below the upper customer specification limit represents a product that is within specification and a process that is in control.
D.  A data observation that falls at a value equal to two standard deviations above the mean represents a product from a process that is in control.

 

Please also briefly explain.

Thank you very much.

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vijjay

Wed, 02/10/2010 - 17:47

A.  The upper customer specification limit represents the project's quality metric for the work performance information. -->No probs
B.  A data observation below the lower control limit yet above the lower customer specification limit represents a process that is out of control and a product that is within specification. -->correct
C.  A data observation that is above the lower control limit and below the upper customer specification limit represents a product that is within specification and a process that is in control. --> within specs but not sure in control
D.  A data observation that falls at a value equal to two standard deviations above the mean represents a product from a process that is in control. --> Info is not sufficient. if spec / control is less than 2 SD then its false. seems less correct statement.

ChandraR

Wed, 02/10/2010 - 18:51

The best choice is A.

Control Chart is used to assess the robustness of your process and after several observations of data, you find the Mean (X bar) and Std. Dev (Sigma).  Your process is considered robust for all data points within  ' Xbar +/-  3* Sigma.'  You don't focus much on customer  upper and lower specification limits as a Quality/Process Engineer.  That is Design Engineer's responsibility to design the product not to exceed the customer upper specification limit.  If you have more than 7 consecutive measurements are either below the mean or above the mean but they are all well within the customer upper spec limit- do you consider everything is fine and well under control?  Absolutely NOT!  Customer upper spec. limit is for product acceptance and control chart limits and all the data plotted in it are meant for Process robustness, consistency and identifying inherent weaknesses in the process- (if left uncorrected- can lead to catastrophic results- sooner or later)  Choice D is within the allowed 3 sigma limits from the mean and is OK, Choice B is an absolute OK statement.  Choice A relies and focuses heavily on the upper spec limit - which is least TRUE among all the choices.

 

Chandra

winthur

Thu, 02/11/2010 - 02:06

this came out for my exam... apparently i fail my first exam :(

ChandraR

Thu, 02/11/2010 - 02:17

What guarantee is there that my interpretation is fool proof and I am 100% correct?  I just expressed my thought process and approach.  If somebody else in this forum comes up with a better explanation and a better choice, I could be proved wrong.  Generally we can confidently eliminate 2 choices and we may fall into the trap when picking up 1 out of the other 2 .  There are at least 39% fail in the PMP exam ( assuming 61% is the typical pass rate).  So you never know.  Also you can afford to pick 39% wrong answers and still pass the exam. 

 

Chandra

pkukilla

Thu, 02/11/2010 - 04:49

Please can you tell the source of this question?

pradeepsodha

Thu, 02/11/2010 - 10:04

My analysis of this Q is 

A.  The upper customer specification limit represents the project's quality metric for the work performance information.

Not correct. Control limit represents (which should be more stringent than customer specification) project quality matric not customer specification limits.


B.  A data observation below the lower control limit yet above the lower customer specification limit represents a process that is out of control and a product that is within specification.

This is correct.


C.  A data observation that is above the lower control limit and below the upper customer specification limit represents a product that is within specification and a process that is in control.

Same as B and it is correct.


D.  A data observation that falls at a value equal to two standard deviations above the mean represents a product from a process that is in control.

As per PMBOK, "for repetitive processes, the control limits are generally +/-3 Sigma". Here question doesn't specify process type and specification limits. So if customer specification limit is +/- 2 sigma, this is correct.

Pradeep

cplusplus

Thu, 02/11/2010 - 15:13

In a control chart used in the Perform Quality Control process, which of the following statements is LEAST true?

 

A.  The upper customer specification limit represents the project's quality metric for the work performance information.
B.  A data observation below the lower control limit yet above the lower customer specification limit represents a process that is out of control and a product that is within specification.
C.  A data observation that is above the lower control limit and below the upper customer specification limit represents a product that is within specification and a process that is in control.
D.  A data observation that falls at a value equal to two standard deviations above the mean represents a product from a process that is in control.

The data observation described in 'C' may be within specification, but the data point might actually be over the upper control limit. Therefore, the process may not be in control as answer 'C' states. Answers 'A', 'B', and 'C' are true.

pkukilla

Thu, 02/11/2010 - 16:08

Cplus, Please can you tell where the source of question is?

ChandraR

Thu, 02/11/2010 - 18:26

Answer 'C' is least TRUE only under the assumption/possibility that the system is out of control.  What if the system is well in Control? 

In Choice 'A' we don't make assumptions.  We set the Process Control Limits based on the process 'mean' and standard deviation and our least worry is the customer upper specification (We - as a QC personnel-can't do anything about it by tweaking our process because Design is the main Driver for meeting the spec. Process is the main driver for variation from part to part.  Why only few Toyota brakes are faulty and needed a large scale recall?  All of them should have met the customer upper spec to get out of the plant - In general all the critical specs are 100% inspected.  Upper spec does not reveal inherent weaknesses) So when you present the Control Chart, you look for trends (whether no consecutive data points are above mean or below- that is an indicator of weakness in the process), look for whether all the data points are within mean+/- 3 sigma limits.  So you never really bother for the customer upper and lower specs when presenting Control Chart.  I still feel that 'A' is least TRUE among all the 4 choices and especially compared to choice C.  Is this also from Andy Crowe's book like the last time question (regarding 'who are all stake holders') you presented to the forum?

 

Chandra