To illustrate the use of CUSUM in the laboratory, this example uses daily control values for glucose testing. In the example, laboratory testing is not performed on weekends, explaining the lack of data on days 1, 7, and 8.
- First, list daily control values under "daily results."
- Then, calculate the mean by using formula A.
- Next, 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 then plot our data points on the Levey-Jennings chart.
- Formula D helps us to 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.
