How is the mean value for a Shewhart plot determined?

• A. By averaging the first scan results.
• B. By scanning the phantom multiple times and calculating the average.
• C. By using a single reference scan.
• D. By consulting historical data.

Answer: (B)

The mean value for a Shewhart plot is determined by scanning the phantom multiple times and calculating the average. This method ensures that the mean is based on a robust dataset that accounts for any variations in the measurements, which can arise from factors such as equipment calibration, the condition of the phantom, and environmental conditions. By taking multiple scans, any outliers or anomalies can be minimized, resulting in a more accurate representation of the typical measurement.

This averaging process is crucial in quality control settings, as it helps establish a control limit that can be used to monitor ongoing performance. By relying on data from multiple scans, it is possible to detect shifts in the mean over time, which could indicate potential issues with the equipment or changes in the measurement technique. Overall, this method of determining the mean adds reliability and validity to the analysis of the data in a Shewhart plot.