8.2 - Accuracy, Precision & Error
Accuracy and precision
Accuracy and precision sound alike, but a set of results can have one without the other:
| Term | What it means |
|---|---|
| Accurate | Close to the true value |
| Precise | The repeated readings are close to each other (a small spread) |
Precise doesn't mean accurate
- A precise result is not always accurate.
- Example: weigh a 50.0 g mass on a balance that hasn't been set to zero, and you might read 52.0 g, 52.1 g, and 52.0 g.
- Those readings are precise because they barely differ - but none is accurate, because every one is about 2 g too high.
Resolution of an instrument
- An instrument that reads to more decimal places, such as a balance reading to 0.01 g rather than 0.1 g, has a higher resolution - the smallest change it can detect.
- Say "higher resolution" for an instrument, not "more precise". Precise describes a set of readings that cluster closely, not a single reading.
Repeatable and reproducible
- Both terms describe results that come out similar when the investigation is done again.
- The difference is who repeats it, and what stays the same.
The two terms
- Repeatable - You get similar results when you repeat the investigation yourself, with the same method and the same equipment.
- Reproducible - Someone else gets similar results, or you get them using a different method or different equipment.
Reproducible results give more confidence than repeatable ones, because a similar result from a different set-up rules out a fault peculiar to one person or one piece of equipment.
Random and systematic error
Two kinds of error affect measurements, and each one does something different:
- Random error - An unpredictable difference between readings, for example from slight differences in judgement when reading a scale. Reduce it by taking repeats and finding the mean.
- Systematic error - A fault that shifts every reading by the same amount in the same direction, such as a balance that is not set to zero. Taking a mean does not reduce it, because every reading is wrong in the same way.
Linking error to precision and accuracy
- Random error scatters your readings, so it harms precision - taking repeats and a mean reduces the effect of random error on your answer.
- Systematic error shifts every reading the same way, so it harms accuracy - no amount of repeating fixes it.
- So a set of results that is precise but not accurate points to a systematic error.
TermDefinition
AccurateDescribes a measurement that is close to the true value.
PreciseDescribes repeated measurements that are close to each other, with very little spread about the mean.
Resolution (measuring instrument)The smallest change that an instrument can detect.
RepeatableDescribes results that are similar when the same person repeats the investigation with the same method and equipment.
ReproducibleDescribes results that are similar when someone else repeats the investigation, or when a different method or equipment is used.
Random errorAn error that makes repeated readings vary unpredictably, spreading them either side of the true value.
Systematic errorA fault that shifts every reading by the same amount in the same direction.
Key points to remember
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