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
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.
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.
Select Defects as the
Numeric Data Variable (Y) >>, N as the
Subgroup Column
(Size) . Click OK. The resulting
U-Chart is shown:
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.
Click OK. The resulting
U'-Chart is shown:
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.
