Data Analysis P-Charts

  • Thread starter Thread starter Guest
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G

Guest

How do you create p-charts using upper control limits
(UCL) and lower contril limits (LCL) using the standard
deviation..?
 
A p-chart is just a line chart. You can indicate the control limits
with added lines, per the examples here:

http://www.geocities.com/jonpeltier/Excel/Charts/ComboCharts.html#AddLine

I would use the point-and-error-bar method, with the point to the right
of the chart, and use a "Show Values" data label on the points.

I also have a poorly documented Dynamic Control Chart on my site that
you might be able to hack to meet your needs:

http://www.geocities.com/jonpeltier/Excel/Charts/statscharts.html#CtrlCht

- Jon
 
A p-chart is a line chart plotting a percentage. Usually this is a
percentage defective. For example, you inspect 250 patient records and you
find 32 of them contain errors. In this case your number inspected is 250,
your number defective is 32, and you would plot (on your line chart) the
percentage or 12.8%. You do this for several "rows" or records. Next, you
compute control limits like this:

Upper limit = pbar + 3 * SQRT(pbar(1-pbar)/nbar)
Lower limit = pbar - 3 * SQRT(pbar(1-pbar)/nbar)

pbar = the average of ALL your percentages
nbar= the avarage of ALL your number inspected values

Your line chart will then contain 4 lines:

Upper limit
the percentage line plot
Lower limit
pbar

If your number inspected changes drastically across all your records, you
should consider computing limits FOR each record - in this case the upper
and lower limits will not be straight lines.

Good luck
http://www.qualityadvisor.com
 
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