3.1.2. Control limits with different sample sizes
Four types of control charts, accounting differently for the sample size variations, were compared and considered (see Appendix 1 in Supplemental Materials). The “Weighted Mean of Variable-Width Control Limits” was deemed the most appropriate:
Pm nUCL i1⁄41i i
UCL1⁄4 Pm suchthatni !1foralli i1⁄41 ni
The “Weighted Mean of Variable-Width control Limits” had the lowest UCLs of the four methods considered when analysing vari- able sample sizes. Low UCL is preferable since it limits the risk of missing a trend or an event of potential significance.