1.12 Precision and Accuracy
A physical measurement should be both precise and accurate. These are two separate concepts and need clear distinction. Generally, precision of a measurement refers to how close together a group of measurements actually are to each other. Accuracy of a measurement refers to how close the measured value is to some accepted or true value.
A classic illustration is a target or bullseye hit by arrows. To be precise, arrows must hit near each other, and to be accurate, arrows must hit near the bullseye. Consistently hitting near the centre of the bullseye indicates both precision and accuracy.
When these concepts are applied to measurements, precision is determined by the instrument being used for measurement. The smaller the least count, the more precise the measurement. A measurement is accurate if it correctly reflects the size of the object being measured. Accuracy depends on fractional uncertainty in the measurement. The smaller the size of the physical quantity, the more precise the instrument needed. The accuracy of a measurement is reflected by the number of significant figures; the larger the number of significant figures, the higher the accuracy.
Table 1.7: Some Timing Devices
| Type of clock/watch | Use and accuracy |
|---|---|
| Atomic clock | Measures very short time intervals of about 10⁻¹⁰ seconds. |
| Digital stopwatch | Measures short time intervals (in minutes and seconds) to an accuracy of ±0.01 s. |
| Analogue stopwatch | Measures short time intervals (in minutes and seconds) to an accuracy of ±0.1 s. |
| Ticker-tape timer | Measures short time intervals of 0.02 s. |
| Watch/Clock | Measures longer time intervals in hours, minutes, and seconds. |
| Pendulum clock | Measures longer time intervals in hours, minutes, and seconds. |
| Radioactive decay clock | Measures (in years) the age of remains from thousands of years ago. |