In a Cusum chart, which axis represents the run number?

• A. X - run number
• B. Y - mean
• C. X - sample number
• D. Y - standard deviation

Answer: (A)

In a CUSUM chart, you’re tracking how the cumulative sum of deviations from the target changes as you move through measurements in their collected order. The horizontal axis represents the run number, the sequential index of each observation, because the chart is about how the process deviates step by step over time. The vertical axis shows the cumulative sum itself, not the data’s mean or its variability, so it reflects whether the process tends to drift above or below the target as more samples are added. Therefore, the run number goes on the X-axis, while the Y-axis displays the accumulated deviation.