CUSUM Example: Plotting Control Data
- To illustrate the use of CUSUM in the laboratory, we'll use 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, 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.