| T O P I C R E V I E W |
| CalSchrotenboer |
Posted - 29 Nov 2012 : 11:36:51
We would like to display a single Cpk and Ppk value for our entire data set in an X-Bar R chart. The example displayed on Page 171 of your QCSPCChart for WPF help file shows a Cpk and a Ppk value corresponding to each data point. We would prefer instead to display the aggregate of all of these individual values. Our desire is consistent with the analysis of Cpk and Ppk found in http://blog.minitab.com/blog/michelle-paret/process-capability-statistics-cpk-vs-ppk
If possible, we would like to display the aggregate Cpk and Ppk values in lieu of the table optionally shown above the chart. Can we display custom labels ("Cpk: " and "Ppk: ") along with the corresponding aggregate values in this area?
|
| 6 L A T E S T R E P L I E S (Newest First) |
| sakamali |
Posted - 02 Aug 2013 : 07:38:20
Hi,
Thanks for notice that standart deviation is coming from entirety of data. Now, all of calculations are correct.
Best Regards. |
| quinncurtis |
Posted - 01 Aug 2013 : 11:14:06
The formulas used in all of the Process Capability (C)and Process Performance (P) ratios are found starting on page 166 of the QCSPCChartNet manual. The main difference between the two is that the Process Capability ratios use the estimate of the sigma = (Rbar/d2), and the Process Performance ratio uses the sigma calculated using a sample standard deviation on the sample population as a whole. We calculated the sample standard deviation for the entirety of your data, to be 1.7798151408.
Pp = (35-27)/(6 * 1.7798151408 ) = 0.749141471
Pl = (20.84-27)/(3*1.7798) = -1.54
Pu = (35-20.84)/(3*1.7798) = 2.65
Seems correct to us.
You USL of 35 and USL of 27 seem invalid for a process with a mean of 20.84. Something is wrong. If the process is out of control, then the Pp and other P values are meaningless according to Douglas Montgomery (Introduction to Statistical Quality Control).
Quinn-Curtis Customer Support |
| sakamali |
Posted - 01 Aug 2013 : 06:03:28
Hi,
We use Quinn-Curtis SPC Variable Control Charts. I couldnt reach your web site and forum, so I sent my question via e-mail, too.
I have these values in double array:
[0][1][2][3][4]
--------------
20 22 21 23 22
19 18 22 20 20
25 18 20 17 22
20 21 22 21 21
19 24 23 22 20
22 20 18 18 19
18 20 19 18 20
20 18 23 20 21
21 20 24 23 22
21 19 20 20 20
20 20 23 22 20
22 21 20 22 23
19 22 19 18 19
20 21 22 21 22
20 24 24 23 23
21 20 24 20 21
20 18 18 20 20
20 24 22 23 23
20 19 23 20 19
22 21 21 24 22
23 22 22 20 22
21 18 18 17 19
21 24 24 23 23
20 22 21 21 20
19 20 21 21 22
And I use USL and LSL values :
spcChart.ChartData.ProcessCapabilityLSLValue = 27;
spcChart.ChartData.ProcessCapabilityUSLValue = 35;
According to formulas, Cp, Cpk, Cpl, Cpm and Cpu are calculated correct.
For example Cp formula is : (USL LSL) / [6 * (RBar/d2)]
: (35 27) / [6 * (3,48/2,326)]
: 8 / 8,97 = 0,89186176142697881828316610925307 ~= 0,891 #61664; This is last value of Cp. And it is true.
But, Pp, Ppk, Pl, and Pu values are calculated wrong (In my opinion).
Are there any parameters to calculate the values? And may I forget them?
In this sample;
How do you calculate Pp, Ppk, Pl and Pu values?
Thanks in advance.
Regards. |
| quinncurtis |
Posted - 29 Nov 2012 : 16:49:46
You can place text in a footer of the chart, which is at the bottom.
spcChart.PrimaryChart.Footer.TextFont = new ChartFont(SPCChartBase.DefaultChartFontString, 12, FontStyles.Normal, FontWeights.Normal);
spcChart.PrimaryChart.Footer.TextString = "XXXX";
spcChart.PrimaryChart.Footer.ChartObjEnable = ChartObj.OBJECT_ENABLE; |
| CalSchrotenboer |
Posted - 29 Nov 2012 : 15:30:16
Thank you for your explanation. I believe that it would be fairly easy for us to display the final values for Cpk and Ppk in our application outside the QC chart. However, this information would not be automatically picked up by the QC printing routine. Based on some test printing which I have performed, it would appear that there generally will be some unused space on the printed page below the chart. Is there a way for us to insert any other data into that space? |
| quinncurtis |
Posted - 29 Nov 2012 : 13:13:32
The software does not have the option of displaying a single aggregate value for the Cpk or Ppk values. You can consider the Cpk and Ppk values for the last sample subgroup to be the aggregate value taking into account all previous sample subgroups. Our display method is actually much better than what you reference, since ours will show the trend of process capability values over time, and it will also show when the values stabilize.
You can retrieve the most recent values using code similar to below:
spcChart.ChartData.AddProcessCapabilityValue(SPCProcessCapabilityRecord.SPC_CPK_CALC);
spcChart.ChartData.AddProcessCapabilityValue(SPCProcessCapabilityRecord.SPC_PPK_CALC);
.
.
.
double cpk = spcChart.ChartData.GetProcessCapabilityValue(0, spcChart.ChartData.CurrentNumberRecords - 1);
double ppk = spcChart.ChartData.GetProcessCapabilityValue(1, spcChart.ChartData.CurrentNumberRecords - 1);
The array index (0 and 1 in this case) used to access the process capability value is the order in which the item was added to the chart using AddProcessCapabilityValue. The last sample subgroups value can be retrieved using the spcChart.ChartData.CurrentNumberRecords - 1 index.
There are no custom display options like you describe. If it is critical to your application, we could write some custom code which would do what you describe. If you send us a mockup picture of what you want we can give you a quote. |
|
|
