User (Legacy) Posted June 10, 2004 Report Share Posted June 10, 2004 Hi- I'm using both Cpk nad Ppk calculations in a control chart. However, I get the same value for each calculation. The difference in these two calculations are the way that the standard deviation is calculated. Cpk uses an estimated sigma => Ppk uses the calculated sigma => I understand that these values may be close for processes in control. However, I have ran the results for each out to 9 decimal places, and they are identical. Is there something I need to change to calculate these values? I've included my code below. double cpk = statistics1.Calculators[0].Get(Analysis.Cpk); double ppk = statistics1.Calculators[0].Get(Analysis.Ppk); StudyCustom Cpk = (StudyCustom) statistics1.Studies.AddTitle("Cpk"); Cpk.Value = Convert.ToDouble(cpk.ToString("0.000000000")); Cpk.Indented = true; Cpk.Bold = false; Cpk.Visible = false; Cpk.Decimals = 9; StudyCustom Ppk = (StudyCustom) statistics1.Studies.AddTitle("Ppk"); Ppk.Value = Convert.ToDouble(ppk.ToString("0.000000000")); Ppk.Indented = true; Ppk.Bold = false; Ppk.Visible = false; Ppk.Decimals = 9; Thanks, Scott Link to comment Share on other sites More sharing options...

Software FX Posted June 10, 2004 Report Share Posted June 10, 2004 If you are drawing a Control chart, The Estimated Un-Biased Sigma will used to calculate the Cpk while the Biased Sigma will be used to calculate the Ppk. If no control chart is being plotted, a biased sigma is used for both calculation as the control limits are undetermined, the formula you attached corresponds to Sigma for an X-Chart (well actually that divided by Sqrt(n)). So, if you are plotting a Control chart you should see different numbers for Cpk and Ppk. If you are not plotting a control chart you will need to assign the value of Sigma yourself based on what type of Control chart you want to see it for. If you have a Sigma value for an "in-control" data, you can also supply it to the Spc chart by doing: statistics1.Gallery.XChart.Sigma = <in control value>; You can also use the SaveState-LoadState feature to calculate Sigma based on "In Control Data". For more information check the Docs. under "SPC Charts - Limit Values" -- FP Software FX Attachments.zip Link to comment Share on other sites More sharing options...

User (Legacy) Posted June 10, 2004 Author Report Share Posted June 10, 2004 Dear FP, Thanks for the quick response. I am currently using a simple line chart. I would rather use the chart that will automatically determine both values correctly. I assume that the x-chart is what I need to be using. I have had problems using this chart, as it only plots a single a point for each value in my series (I'm using a single series only). When I look at the point value, it is the average for all values in my series. I have attempted to assign a series to each point, but have been unsuccessful. I'm not sure what the correct way is to pass the data in to an x-chart. Can you tell how the x-chart works or point me to the instructions in the API to get the chart to plot individual sample values? Thanks, Scott Dozier "SoftwareFX Support" <noreply@softwarefx.com> wrote in message news:BesMf9xTEHA.988@webserver3.softwarefx.com... > If you are drawing a Control chart, The Estimated Un-Biased Sigma will used > to calculate the Cpk while the Biased Sigma will be used to calculate the > Ppk. > > If no control chart is being plotted, a biased sigma is used for both > calculation as the control limits are undetermined, the formula you attached > corresponds to Sigma for an X-Chart (well actually that divided by Sqrt(n)). > > So, if you are plotting a Control chart you should see different numbers for > Cpk and Ppk. If you are not plotting a control chart you will need to assign > the value of Sigma yourself based on what type of Control chart you want to > see it for. > > If you have a Sigma value for an "in-control" data, you can also supply it > to the Spc chart by doing: > > statistics1.Gallery.XChart.Sigma = <in control value>; > > You can also use the SaveState-LoadState feature to calculate Sigma based on > "In Control Data". For more information check the Docs. under "SPC Charts - > Limit Values" > > -- > FP > Software FX > > Link to comment Share on other sites More sharing options...

Software FX Posted June 11, 2004 Report Share Posted June 11, 2004 X charts work by making an average of each sample and plotting it. A good explanation of x-chart can be found at: http://www.itl.nist.gov/div898/handbook/pmc/section3/pmc321.htm Normally, each sample will consist of several measurements, each Sample should a series in your original data. An X-Chart (Actually is X with two bars on top meaning "The Average of the Averages"), plots the mean of each sample as one point, so it will plot one point for each series in your original data, regardless of how big your samples are. If you only supply ONE value per sample, we will not be able to calculate the estimated Sigma as the range will be 0. So you will need to provide us with one. -- FP Software FX Link to comment Share on other sites More sharing options...

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