Validation2011
Starting to get Involved
Hi All,
Hope you can offer some guidance!
I am currently trying to calculate capability on the results of 3 batches of product.
I am using the Stat-Quality Tools-Capability Analysis-Normal method in minitab.
I want to treat the 3 sets of results as 3 separate groups. Each of the groups is passing capability testing on an individual basis but because the particular feature is quite hard to accurately control all 3 groups are located in different positions within the specification range and when they are combined we fail the capability analysis.
I see in minitab that the default setting is "Pooled Standard Devaition" but when i calculate the capability on the 3 groups of data it seems to automatically combine them all into 1 group and work out the standard deviation for the 1 group rather than pooled standard devation for the 3 groups.
Specfication range is -0.0025 to 0.0025. When i calculate capabilty in minitab i get PpK = 2.39.
When i calculate the capabilty manually in excel using standard deviation (StDev) i also get 2.39.
When i calculate the capability manually in excel using a pooled standard deviation (sp=[((n1-1)s1^2 + (n2-1)s2^2 + (n3-1)s3^2) / (n1 + n2 + n3 - 3))]^1/2 i get 3.10.
Can someone shed some light on exacly what i am doing wrong when calculating the capability in minitab?
Many Thanks
Hope you can offer some guidance!
I am currently trying to calculate capability on the results of 3 batches of product.
I am using the Stat-Quality Tools-Capability Analysis-Normal method in minitab.
I want to treat the 3 sets of results as 3 separate groups. Each of the groups is passing capability testing on an individual basis but because the particular feature is quite hard to accurately control all 3 groups are located in different positions within the specification range and when they are combined we fail the capability analysis.
I see in minitab that the default setting is "Pooled Standard Devaition" but when i calculate the capability on the 3 groups of data it seems to automatically combine them all into 1 group and work out the standard deviation for the 1 group rather than pooled standard devation for the 3 groups.
Specfication range is -0.0025 to 0.0025. When i calculate capabilty in minitab i get PpK = 2.39.
When i calculate the capabilty manually in excel using standard deviation (StDev) i also get 2.39.
When i calculate the capability manually in excel using a pooled standard deviation (sp=[((n1-1)s1^2 + (n2-1)s2^2 + (n3-1)s3^2) / (n1 + n2 + n3 - 3))]^1/2 i get 3.10.
Can someone shed some light on exacly what i am doing wrong when calculating the capability in minitab?
Many Thanks
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