Precision
Measurements & Uncertainties - OCR A-Level Physics
Key Definition
Precision
How close repeated measurements are to each other. A precise set of results has a small spread (low random error).
How close repeated measurements are to each other. A precise set of results has a small spread (low random error).
Key Definition
Accuracy
How close a measurement is to the true or accepted value. High accuracy means low systematic error.
How close a measurement is to the true or accepted value. High accuracy means low systematic error.
- Results can be precise but not accurate: all clustered together but far from the true value (systematic errorAn error that shifts all readings by the same amount in the same direction. Cannot be reduced by repeating measurements.).
- Results can be accurate but not precise: scattered widely but centred on the true value.
- The ideal is both precise AND accurate.
- Think of a dartboard: $precisionHow close repeated measurements are to each other. A precise set of results has a small spread (low random error). = darts close$ together, $accuracyHow close a measurement is to the true or accepted value. High accuracy means low systematic error. = darts near$ the bullseye.
Key Definition
Resolution
The smallest change in a quantity that an instrument can detect. For example, a ruler has a resolution of 1 mm.
The smallest change in a quantity that an instrument can detect. For example, a ruler has a resolution of 1 mm.
- Higher resolutionThe smallest change in a quantity that an instrument can detect. For example, a ruler has a resolution of 1 mm. does NOT guarantee higher accuracyHow close a measurement is to the true or accepted value. High accuracy means low systematic error. -- you can still have a systematic errorAn error that shifts all readings by the same amount in the same direction. Cannot be reduced by repeating measurements..
- The uncertainty in a single reading from a scale is typically +/- half the smallest division.
- For a digital instrument, the uncertainty is +/- 1 in the last displayed digit.
Examiner Tips and Tricks
- OCR examiners frequently test the precisionHow close repeated measurements are to each other. A precise set of results has a small spread (low random error).-accuracyHow close a measurement is to the true or accepted value. High accuracy means low systematic error. distinction using a dartboard analogy or a table of repeated results.
- You must use the correct term: 'precise' for close repeats, 'accurate' for close to true value.
- Mixing them up loses marks every time.