Like the Westgard Rules, the Cumulative Summation Limit or Rule (abbreviated as CUSUM) has different approaches.
- The CUSUM type used on the following pages is more sensitive to systematic than random error. Nevertheless, it does provide an easy means to detect impending problems.
- CUSUM is calculated on worksheets like the one below.
- Basically, CUSUM works in the following manner: a decision limit is predetermined (See section E. on the right side of the chart, where the CUSUM limit is defined as SD x 2.7), and when the CUSUM of control observations exceeds this limit, one must look for error in the testing process.
- The right side of the worksheet is used to determine the mean, standard deviation (SD), and CUSUM limit.
Note: Be sure to scroll down on each of the pages of this section to view the CUSUM worksheets and their associated QC charts.
