Minitab Cpk calculations results in *

[darcze]

Hello,

I need your help in using Minitab. I am not statistical expert, the thing I am doing was inherited and I need to continue the analysis as I do now.

For our customer we evaluate their forecast accuracy expressed in Cpk.
In minitab I take all their weekly forecast accuracy values expressed in full number with no% so 100% would show as 100.

I put all numbers in one column, then select Stat> Quality Tools > Capability Analysys > Normal....

I select:
Data arranged as single column C1
Subrgoup size C1
Lower spec 50
Upper spec 150

I hit OK and the result I get is Cpk = *

For any other customer I get numbers varying from 0 to 2.
This set of numbers is a firly good forecast accuracy between 90-120% most of the time with a few exceptions. I read Cpk >= 1.33 is the good one, then why does not Minitab show me a value like this? How di I interpret the * and what should I change to actually get Cpk for the given set of numbers?

The numbers I am using are below:
125
123
123
91
91
106
103
117
111
106
110
123
103
91
90
103
115
98
123
80
83
84
113
96
95
84
100
105
139
108
117
94
0
95
121


Please help, it stops my entire analysis and I do not know how to move forward.

 

[Statistical Steven]

Your subgroup size should be 1 I believe not C1

 

[darcze]

Thank you Steven, Cpk 0.97 looks much better and can be comparable with other values.

I wonder why C1 in subgroup did work for all other products I was measuring for and refused to work on the above example.

Will check values again, thank you one more time !

 

[supreecha]

Minitab Cpk =0.70 ( in excel) , Ppk =0.97 ,Mean=101.86 ,Stdev=22.758
Overall Performance =28,558 ppm in this sample.

 

[Miner]

Steven is correct. The only time you should use a column to designate the subgroup size is when you have varying sizes. For example, you could have a column called Month and enter 1 1 1 1 2 2 2 2 2 3 3 3 3 4 4 4 4, and Minitab would group the 1s into the first subgroup, 2s into the second and so forth.

Another point to remember is how the data were collected affects what you enter.

Data collected one at a time: Subgroup size = 1
Data collected in subgoups of 5: Subgroup size = 5
Data collected all at once: Subgroup size = 35

What you enter will affect the Potential (Within) Capability results (Cp, Cpk).