RE: Control Charts for Alpha Counters

["MacLellan, Jay A" <jay.a.maclellan@pnl.gov>]

Sandy Perle wrote "In my opinion the lab should use a true Shewart Control
Chart, obtain 
at least 30 independent data points, determine the true UCL and LCL, 
based on the appropriate equations, and then use these Control Limits 
to determine when the process is out of control. A true Shewart 
Control Chart is based on normal tendency, which results in 99.97% of 
true normal data falling within +/- 3 sigma."  

This is a good plan if your routine count time is short enough to allow you to
collect 30 background measurements.  If you are using a long routine count time,
it isn't always practical.  Another caveat concerns the probability distribution
for the counts.  If you are characterizing a low background system, the
distribution is Poisson, not normal.  And if you only collect a few counts per
background measurement, you don't have a good estimate of the mean.  All these
factors should be included in your control chart.

Jay A. MacLellan, CHP
Radiation and Health Technology
Pacific Northwest National Laboratory
Phone:  509-376-7247
Fax:      509-376-2906
Email:   jay.a.maclellan@pnl.gov

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