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Ppk analysis - number of digits

Dear jmp users, 

I need to check Ppk for a response with Lower and Upper Specification Limits of 1.5 and 1.8, respectively. Numbers are given with one digits after the coma.  

All my responses were within this range (many values were 1.8 which is acceptable in our process), however the Ppk was low (0.25) and the histogram showing values of 1.8 out of specifications when adjusting the USL to 1.8 : 

 

If I try to adjust it to 1.85 (considering also values between 1.81-1.84 which are also acceptable), the Ppk becomes higher :  

 

 

My question is : which option should I consider ? the first or the second ? 

 

Thanks !

I 

 

GraphicsRe: Ppk analysis - number of digits

The purpose of calculating process capability is to evaluate the performance (output) of the process. You start with specifications that define quality based on required performance, not actual performance. You are using this method in reverse. You are using the performance to define the specification. First determine the levels of APO0740 Guanidine beyond which the output will not be acceptable. Then assess the process capability.

 

Your process is not centered, so the capability index will be low, as it is designed to penalize an off-target process.