Hi, Im a little confused can anyone help.

If I need to maintain a 95% reliability on parts dose that mean I need to maintain a true 0.66Cpk.

You are really confused

, I think what you want is the lower limit of the Cpk estimation to be 0.66 at 95% of confidence.

note added: when you obtain the value of Cpk from a data set, the value is just an estimate for such sample, and there are some formulas to determine the range in wich the real value (for such sample) must lay, so it's a minimum and a maximum value for such interval where the real value must be, such interval is calculated in function with the %confidence specified for such calculus.

To answer this question, sample size is needed, and you didn't provide such information, so I will give you the way to do it, but you will need to add VBA code in Excel. It will give the value of Cpk needed to obtain a minimum value at 95% of 0.66

HTML:

```
Function CpkNotLessThan(Cpk_Min As Single, Conf100 As Byte, Sample_Size) As Single
Dim N As Long, BIG as boolean, Cpk_Obj as Single, Delta as Single
Cpk_Obj = Cpk_Min: Delta = 0.5: BIG = False
Do
DIF = Cpk_Min - (Cpk_Obj - Application.WorksheetFunction.NormSInv((1 - (1 - Conf100 / 100) / 2)) * (1 / (9 * Sample_Size) + (Cpk_Obj) ^ 2 / (2 * Sample_Size - 2)) ^ 0.5)
If Abs(DIF) < 0.0001 Then GoTo 1
If DIF > 0 Then
If BIG Then Delta = Delta / 2
BIG = False
Cpk_Obj = Cpk_Obj + Delta
Else
If Not (BIG) Then Delta = Delta / 2
BIG = True
Cpk_Obj = Cpk_Obj - Delta
End If
Loop Until False
1: CpkNotLessThan = Cpk_Obj
End Function
```

after writing the code and enabling VBA Macros, in an Excel Sheet, you will need to write

**=CpkNotLessThan(0.66,95,50)**, for sample size of 50.