The Cusum plot is defined as what?

• A. A sum of daily differences between actual and expected values
• B. A histogram of results
• C. A plot of patient age vs result
• D. A scatter plot

Answer: (A)

A Cusum plot shows the cumulative sum of deviations from a target value over time. At each time point you take the difference between what you observed and what was expected, and add that difference to the running total. This creates a curve that highlights small, persistent shifts in the process because even tiny, consistent deviations accumulate and push the curve away from zero. If the process is truly centered on the target, the deviations balance out and the cusum stays near zero. This is different from a histogram, which shows how results are distributed; a plot of age versus result or a scatter plot would depict relationships between variables rather than cumulative deviation over time.