Harish
03-08-2008, 07:49 AM
All,
This is a question related to control chart understanding for analyzing stablity and capability of Defect Density (DD).
We have a project where USL & LSL for DD is 0.20 and 0.0
For a good project where defects are low, the mean of the actual DD is 0.02 with a standard deviation of 0.01.
Thus the UCL and LCL at 3 sigma is 0.05 & -0.01 respectively.
From a software project management, the project is doing fine, however, from a statistical perspective the LCL is below LSL & hence the subprocess is not stable (and also not capable)
Now, from a statistical perspective, if the mean was around 0.05 having the same standard devation of 0.01 as the earlier example, it would have been both stable & capable.
Alternatively, we could have changed the LSL for DD to say -0.05 but that is ridiculous as defect density cannot be below zero.
So the question is - "The project manager says his process is both stable & capable, but the quality person says that LCL is below LSL and the process is not stable and hence not capable. Who is right - the Project Manager or the Quality person?."
This is a question related to control chart understanding for analyzing stablity and capability of Defect Density (DD).
We have a project where USL & LSL for DD is 0.20 and 0.0
For a good project where defects are low, the mean of the actual DD is 0.02 with a standard deviation of 0.01.
Thus the UCL and LCL at 3 sigma is 0.05 & -0.01 respectively.
From a software project management, the project is doing fine, however, from a statistical perspective the LCL is below LSL & hence the subprocess is not stable (and also not capable)
Now, from a statistical perspective, if the mean was around 0.05 having the same standard devation of 0.01 as the earlier example, it would have been both stable & capable.
Alternatively, we could have changed the LSL for DD to say -0.05 but that is ridiculous as defect density cannot be below zero.
So the question is - "The project manager says his process is both stable & capable, but the quality person says that LCL is below LSL and the process is not stable and hence not capable. Who is right - the Project Manager or the Quality person?."