# QualityMagazine - March 2012

## How To Select Control Charts Using Sigmaxl's Control Chart Selection Tool: Example - P' (Laney) Chart

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The Control Chart Selection Tool in SigmaXL makes it easy for you to select the correct statistical process control chart depending on your data type, subgroup/ sample type and size.
We will use an example of discrete data with columns: Defective Units and Subgroup Size (N):
Typically one would use a P-chart for defective units with varying subgroup size, but in this case, results in a chart with narrow limits and several data points that appear to be “out-of-control”:
Now we will select SigmaXL’s Control Chart Selection Tool:
Here we see another option for this type of data called the P’ (Laney) chart:
The P’ (Laney) chart shows that the process is actually “in-control”.
P’ (Laney) Control Charts are attribute control charts that should be used when the subgroup/sample size is very large and assumptions are not met. Typically you will see that the control limits do not “look right”, being very tight as we see in the example above. This problem is also referred to as “overdispersion”. This occurs when the assumption of a Binomial distribution for defectives is not valid.
Download a free 30 day trial of SigmaXL to test drive the Control Chart Selection Tool for your data!
For further technical details on the P’ (Laney) chart see: Laney, David B., Improved Control Charts for Attribute Data, Quality Engineering 2002;14:531–7.
By: John Noguera, P.Eng., CTO and Co-founder, SigmaXL, Inc.
www.SigmaXL.com
E-mail: information@SigmaXL.com
1-888-SigmaXL (1-888-744-6295)

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