Cpk calculation vs Ppk

[User (Legacy)]

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

[Software FX]

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

[User (Legacy)]

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

>

>

[Software FX]

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