What does a slope exceeding 45° on a Cusum plot indicate?

A slope exceeding 45° on a Cusum plot indicates that there is a systematic error present. In quality control and process monitoring, a Cusum (cumulative sum) plot is used to detect shifts in the mean of a set of measurements. When the slope of the Cusum plot is greater than 45°, it signifies that the measurements consistently fall in a specific direction, either above or below the expected target value.

This indicates a systematic error, where there is a consistent bias in the measurements rather than random fluctuations. Such a trend would show that the results are not just varying due to random noise but are consistently deviating from the expected results, implying a shift in performance or accuracy of the testing process. This information is useful for clinicians and laboratory technicians as it signals the need for investigation and potentially corrective action to address the underlying issue affecting the results.