Quality Control Procedures, Documentation and Interpretation: Cumulative Summation Limit

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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