The standard needs to be ______ times more accurate than the tool being calibrated.

• A. Three times
• B. Four times
• C. Five times
• D. Two times

Answer: (B)

In calibration, you want the reference standard to have much higher fidelity than the instrument being calibrated. This keeps the standard’s own uncertainty from dominating the comparison, so the calibration mainly reflects the instrument’s true performance rather than the calibrator’s limitations. A widely used rule of thumb is that the standard should be markedly more accurate—roughly four times more accurate than the device under test. With the standard this much more accurate, the combined uncertainty is driven primarily by the instrument under test, making the calibration results more reliable. If the standard were only modestly more accurate, its uncertainty would contribute a larger portion to the total, reducing confidence in the adjustment. A larger margin (five times or more) can work, but four times is a practical, commonly adopted balance. That’s why selecting the standard as four times more accurate is the best choice.