Zero percent means the measurements are equal. A small percentage means they are close compared with their average, while a larger result signals more separation. Whether a value is acceptable depends on the tolerance, uncertainty, or business rule for the subject.
For nonnegative values, the result can approach 200%. If one value is zero and the other is positive, the percentage difference is 200%, because the gap equals twice their average. Always report the two original figures so the relative number does not hide their practical size.
Percentage difference is often used as a quick descriptive comparison, but it is not automatically a test of accuracy or statistical significance. Two instrument readings can be close in percentage terms and still lie outside an allowed tolerance. Conversely, a noticeable percentage may be harmless when measurement uncertainty is large. Compare the result with a limit chosen for the subject, and consult the relevant measurement procedure where one exists. If one number is an accepted or theoretical reference, use percent error instead so the reference remains the denominator. If observations form a time sequence, use percentage change so the earlier value remains the baseline. Choosing the measure before calculating makes the wording of the result much clearer. It also helps another reader reproduce your calculation from the same pair of measurements.
