Experimental uncertainty
Also called Measurement uncertainty, Error bars
Experimental uncertainty is the range within which a measured value plausibly lies, reported as a value plus or minus a range. It is a property of the measurement itself, not evidence that somebody made a mistake.
The AP Physics 1 CED puts it to teachers directly: students need to understand that scientific instruments do not produce exact measurements, and to learn what steps reduce uncertainty. A result of is a complete answer. A result of with no range is an incomplete one.
Reading it off an instrument:
- An analogue scale is usually quoted to half its smallest division, so a metre rule marked in millimetres gives about .
- A digital display is usually quoted to one unit in its last digit.
- Repeating a measurement and spreading the results gives you a better estimate than either.
Random uncertainty scatters readings either side of the true value, and averaging repeated trials shrinks it. Systematic error shifts every reading the same way, from a scale that does not zero or a stopwatch started late, and repeating will never remove it. Only the first kind is what error bars on a graph show.
The exam pays for this. In the AP Physics 1 sample Experimental Design and Analysis question, one of the ten points goes purely to describing a reasonable method of reducing experimental uncertainty, in that case by taking pressure readings at several depths rather than one.