Uncertainty is the interval within which the true value can be expected to lie with a given level of confidence. For example, 20°C ± 2°C at 95% confidence.

• A. The exact result of the measurement.
• B. The range around the measurement where the true value is likely to lie.
• C. The instrument's calibration factor.
• D. The time taken to perform the measurement.

Answer: (B)

Uncertainty is the range around a measured value where the true value is expected to lie, with a specified level of confidence. In the example 20°C ± 2°C at 95% confidence, the true temperature is likely between 18°C and 22°C. This shows that the measurement comes with doubt; we don’t just have a single number, we have an interval that reflects possible variation.

The exact result would be just 20°C, but that ignores measurement error. The instrument’s calibration factor relates to systematic bias, not the interval that expresses uncertainty. The time taken to perform the measurement isn’t what defines the uncertainty—though longer measurements can influence random errors, the interval itself is the uncertainty.