In Shewhart Rules, control limits are based on what percentage of the average values?

• A. 0.5%
• B. 1.0%
• C. 1.5%
• D. 2.0%

Answer: (C)

In Shewhart control charts, control limits are determined based on statistical principles involving the average values of a process, primarily focusing on the standard deviation within control limits. The choice of 1.5% is related to the concept of how many standard deviations can be used to define the control limits, which typically encompass the range within which the majority of values (like 95% under normal distribution) should fall.

1.5% is used in this context to create control limits that are set at a certain distance from the mean, reflecting a balance where there is sufficient room to accommodate natural variability while still effectively detecting any significant deviations that indicate an out-of-control process. The control limits are typically set at ±3 standard deviations from the mean, accommodating for a 99.73% probability in a normal distribution, which is robust and helps to identify any unusual or concerning trends in the data.

Understanding Shewhart control limits and their relation to percentages helps in maintaining quality control in clinical practices like bone densitometry. Implementing these standards allows practitioners to ensure accurate measurements and observational consistency in diagnostic imaging, making 1.5% an essential figure in identifying acceptable variability in bone density measurements.