Hi All,  I working on Process capability study for my project. I have a question on entering the data points. I have taken 25 Months data, wherein 12 Months are zero defects and other months have Defects less than 3. Do I need to include all 25 Months of data or only the 13 months data which has defect numbers. Can someone explain how to calculate the Cp and Cpk for these type of processes.

Hi Arun

 

You will have to use data for all 25 months. There are guidelines on removing a data point while doing analysis, however looking at the description provided by you, I do not feel a need to remove any of the data points. 

 

Cp and Cpk are typically calculated for Continuous Data types, however since your metric i.e. Defects is Discrete, it is better to calculate DPU or DPMO. You may refer our calculator at the below link to calculate the current Sigma Level of the process. Enter the total sample size, the number of total defects found and number of opportunities (if known)

https://www.benchmarksixsigma.com/calculators/sigma-level-calculator-discrete-data-defects/

 

Alternatively you could also run the Poisson Process Capability analysis in Minitab. Path is as follows

 

Stat --> Quality Tools --> Capability Analysis --> Poisson

 

Here you will have to enter data in columns. One for sample size and the other for defect count.

 

Hope this helps.