8.2 - 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 precise result isn't always accurate.
- For example, weighing a 50.0 g mass on a balance that isn't zeroed might give 52.0 g, 52.1 g, and 52.0 g. These readings are precise (they barely differ) but not accurate (they're all about 2 g too high).
Resolution
- The resolution of an instrument is the smallest change it can detect.
- A balance reading to 0.01 g has a higher resolution than one reading to 0.1 g.
- Describe an instrument as having "higher resolution", not as being "more precise" - precision describes how closely a set of readings cluster, not a single reading.
Reliable
- A result is reliable when it comes out much the same each time the investigation is repeated, whoever does it.
- Reliable results give you confidence that a 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 (ignoring any anomalous results).
Random and systematic error
- Random error - An unpredictable difference between readings, such as slight differences in judgement when reading a scale. Taking repeats and calculating a mean reduces its effect.
- 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). Taking a mean doesn't reduce it.
Effects on accuracy and precision
- Random error scatters readings, reducing precision.
- Systematic error shifts every reading the same way, reducing accuracy.
- A set of results that is precise but not accurate points to a systematic error.
Anomalous results
- An anomalous result (or outlier) lies well outside the scatter of the rest of the readings.
- Exclude anomalies when calculating the mean.
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.
Anomalous resultA result that does not fit the pattern of the other results.