Software FX Community

# Cpk calculation vs Ppk

## Recommended Posts

```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);
Cpk.Value = Convert.ToDouble(cpk.ToString("0.000000000"));
Cpk.Indented = true;
Cpk.Bold = false;
Cpk.Visible = false;
Cpk.Decimals = 9;
Ppk.Value = Convert.ToDouble(ppk.ToString("0.000000000"));
Ppk.Indented = true;
Ppk.Bold = false;
Ppk.Visible = false;
Ppk.Decimals = 9;
Thanks,
Scott```
##### Share on other sites

```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

##### Share on other sites

```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
>
>```
##### Share on other sites

```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```

#### Archived

This topic is now archived and is closed to further replies.

×