How Do I Create U' Charts (Laney) in Excel Using SigmaXL?

P' and U' (Laney) Control Charts are attribute control charts that should be used when the subgroup/sample size is very large (i.e. > 5,000) and assumptions are not met. Typically you will see that the control limits do not “look right”, being very tight with many data points appearing to be out-of-control. This problem is also referred to as “overdispersion”. This occurs when the assumption of a Binomial distribution for defectives or Poisson distribution for defects is not valid. Individuals charts are often recommended in these cases, but Laney’s P’ and U’ charts are a preferred alternative.

U'- Charts

1. Open Attribute Data – U’ Defects - Crossley.xlsx. This data is used with permission from Mark Crossley and was used in the example given in reference [1]. Note that there are multiple defects per unit.
2. We will begin with the creation of a regular U-Chart for this data. Select SigmaXL > Control Charts > Attribute Charts > U. Ensure that the entire data table is selected. If not, check Use Entire Data Table. Click Next.
3. Select Defects as the Numeric Data Variable (Y) >>, N as the Subgroup Column (Size) . Click OK. The resulting U-Chart is shown:
   
   
   
   
4. Select Sheet U' Chart Defects - Crossley. Click SigmaXL > Control Charts > Attribute Charts > U’ (Laney). Click Next.
5. Select Defects, click Numeric Data Variable (Y) >>; select N, click Subgroup Column or Size >>. (If we had a fixed subgroup size, the numerical value of the subgroup size could be entered instead of Column N.) Check Tests for Special Causes.
   
   

 



6. Click OK. The resulting U'-Chart is shown:





7. Now we see that the process is actually “in-control.” Laney’s Sigma (Z) is a measure of the overdispersion. See reference [1] for further details.

 

References:

1. Laney, David B., P-Charts and U-Charts Work (But Only Sometimes), Quality Digest, http://www.qualitydigest.com/currentmag/departments/what_works.shtml
2. Laney, David B., Improved Control Charts for Attribute Data, Quality Engineering 2002;14:531–7.
3. M A Mohammed and D Laney, Overdispersion in health care performance data: Laney’s approach, Qual. Saf. Health Care 2006;15;383-384.