Cusum Information and Courses from MediaLab, Inc.
These are the MediaLab courses that cover Cusum and links to relevant pages within the course.
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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 E. 2.7 X SD), 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. | View Page |
| CUSUM Example: Plotting Control Data To illustrate the use of CUSUM in the laboratory, we'll use daily control values for glucose testing.First, we'll list daily control values under "daily results."
Then, we'll calculate mean by using formula A.
Next, we can find the difference from the mean for each result, and square that result for the two relevant columns.
Using all of the squared differences from the mean, we can find the standard deviation using formula B.
Using the mean from formula A and the standard deviation calculations from formulas B and C, we can plot our data points on the Levey-Jennings chart.
Formula D helps us calculate the coefficient of variation (CV), which expresses SD as a percentage of mean value and is more reliable for comparing precision at different concentration levels. The lower the CV the greater the precision.
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| CUSUM Example: Calculating CUSUM Now, we'll calculate the CUSUM value for each daily control. The CUSUM tracks the cumulative distance from the mean, taking into account whether each daily control fell above or below the mean. On March 2, the daily result equaled the control, so the CUSUM is 0. On March 3, the daily control was +3 from the mean, so we add +3 to the previous day's CUSUM, for a total of +3. On March 4, the daily control was -1 from the mean, so we add -1 to +3, resulting in +2. On March 5 and 6, the CUSUM again changes -1 each day, since each control result is -1 from the mean. | View Page |
| CUSUM Example: Exceeding the CUSUM Limit On March 10, the CUSUM is nearing the CUSUM Limit calculated in formula E. This should serve as a warning. While patient results are still "in control," there is a trend indicating that they will soon not be. Sure enough, on March 11, the CUSUM reaches -4, which is the CUSUM Limit, and patient results cannot be accepted as accurate. It is important to observe daily any trends in the CUSUM values so that corrective action can be taken before the limit is reached. | View Page |
| CUSUM and Westgard Rules Looking at the Levey-Jennings chart you will notice that the plots correspond with the Westgard rule 41s. What type of movement and error do you think this reflects? | View Page |