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What is a Cumulative Summation Limit?

  • Like the Westgard Rules, the Cumulative Summation Limit or Rule (CUSUM for short) 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 CUSUM limit is defined as SD x 2.7), and when the CUSUM of control observations exceed 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.