10.2 - Accuracy, Precision & Error
Accuracy and precision
| Term | What it means |
|---|---|
| Accurate | Close to the true value |
| Precise | Repeated readings are close to each other (a small spread) |
A set of results can be precise without being accurate. For example, weighing a 50.0 g mass on a balance not set to zero might give 52.0 g, 52.1 g, and 52.0 g. These readings are precise (they barely differ), but none is accurate (each is about 2 g too high).
Resolution
- Resolution is the smallest change an instrument can detect.
- A balance reading to 0.01 g has a higher resolution than one reading to 0.1 g.
- Use "higher resolution" for an instrument, not "more precise" - precise describes a set of readings that cluster closely, not an instrument.
Reliable
- A result is reliable when it comes out much the same each time the investigation is repeated, whoever does it.
- Reliable results give confidence that the reading isn't a one-off or the result of a mistake.
Making results reliable
Repeat the investigation, check that the readings agree, and calculate a mean.
Random and systematic error
- Random error - An unpredictable difference between readings, such as slight differences in judgement when reading a scale.
- Systematic error - A fault that shifts every reading by the same amount in the same direction, such as a balance not set to zero (a zero error).
How errors link to precision and accuracy
- Random error scatters readings, so it reduces precision. Taking repeat readings and calculating the mean reduces the effect of random error.
- Systematic error shifts every reading the same way, so it reduces accuracy. Taking a mean doesn't reduce systematic error; the method or equipment must be corrected.
- A set of results that is precise but not accurate indicates 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.
ResolutionThe smallest change an instrument can detect.
ReliableDescribes results that come out much the same each time the investigation is repeated.
Random errorAn unpredictable error that makes readings vary above and below the true value, so repeat readings differ from each other.
Systematic errorAn error that makes every reading differ from the true value by the same amount in the same direction.
Zero errorA systematic error caused by a measuring instrument that gives a reading other than zero when the true value is zero.