Beginner · 9 min
Repeating and recording measurements
Use repeated readings to find a mean and describe their spread.
Repeat a reading thoughtfully
Measurements can vary because of small changes in the setup or in how a scale is read. Repeating a measurement helps you notice this variation. Keep the method and units consistent. The mean is the sum of the readings divided by their number. Do not discard a reading simply because it differs from the others; first look for a reason, such as recording the wrong unit.
Worked example
Three length readings are 12.1 cm, 12.3 cm, and 12.2 cm. Mean = (12.1 + 12.3 + 12.2) ÷ 3 = 12.2 cm
Describe the spread
The range of repeated readings is the largest reading minus the smallest. It describes spread, not the difference from the true value. Repeated readings can agree closely and still all be shifted by an instrument's zero error. Record sensible digits based on the measuring tool. Taking a mean does not automatically justify reporting many extra decimal places.
Worked example
Readings of 5.2 s, 5.5 s, and 5.3 s have range 5.5 − 5.2 = 0.3 s. A timer consistently starting late could still give a biased result.
Check a small set of readings
- For the readings 8.1 cm, 8.3 cm, and 8.2 cm, calculate the mean and the range.
- Imagine that the ruler's zero mark is misplaced. Explain why repeating the measurement would not remove that error.