Errors, uncertainty and the language of measurementSpec S5
In short
Read scales to the nearest half division, correct zero errors, tell random from systematic error, and treat results within 10% as equal.
The Combined Science practical papers (Paper 5 and Paper 6) test some measurement skills that are shared with chemistry and physics. You need to read scales carefully, spot and correct errors, judge whether results agree, and use the words for measurement correctly.
Reading scales
- Take readings with the right precision. Where the marks on a scale are far apart, read to the nearest half-scale division. On a thermometer marked every 1 °C, a reading halfway between 24 °C and 25 °C is recorded as 24.5 °C.
- Correct for zero errors. A zero error is when an instrument does not read zero when it should, such as a balance that shows 0.2 g with nothing on it. Subtract the zero reading from every measurement: 5.6 g − 0.2 g = 5.4 g.
- If the end of a ruler is worn, measure from the 1.0 cm mark and subtract 1.0 cm from each reading.
Types of error
| Type of error | What it is | How to reduce it |
|---|---|---|
| Measurement error | The difference between a measured value and the true value. Every measurement has some. | Use an instrument with a finer scale, such as a syringe instead of a beaker |
| Random error | Readings vary unpredictably, some above and some below the true value, for example when judging a colour change or counting bubbles by eye | Repeat the readings and calculate a mean |
| Systematic error | Every reading is wrong by the same amount or in the same direction, for example because of a zero error or a thermometer that always reads 1 °C too high | Check and correct the instrument. Repeating the readings does not remove it |
Uncertainty and the ±10% rule
Uncertainty means you cannot be sure that a value or a conclusion is correct. Causes include readings that are hard to judge (such as a colour end-point), variables that were not controlled, too few repeats, too small a range of values and anomalous results.
At this level, two results count as equal within the limits of experimental accuracy if they differ by no more than 10%. Work out 10% of one value and check whether the other value is inside that range.
Are two results equal within ±10%?
Two students time the same enzyme reaction. One records 50 s and the other records 54 s. Are the results equal within the limits of experimental accuracy?
- 10% of 50 s = 5 s
- The range allowed is 50 s ± 5 s, which is 45 s to 55 s.
- 54 s is inside this range.
Answer: Yes, the results are equal within the limits of experimental accuracy (±10%).
The language of measurement
- True value
- The value that would be obtained in an ideal measurement.
- Measurement error
- The difference between a measured value and the true value of a quantity.
- Accuracy
- A measurement is accurate if it is close to the true value.
- Precision
- How close the measured values of a quantity are to each other.
- Repeatability
- A measurement is repeatable if the same or a similar result is obtained when it is repeated under the same conditions, by the same method, in the same experiment.
- Reproducibility
- A measurement is reproducible if the same or a similar result is obtained under different conditions, by a different method or in a different experiment.
- Validity
- An experiment is valid if it tests what it says it will test: a fair test in which only the independent and dependent variables change and the controlled variables are kept constant.
- Range
- The maximum and minimum values of the independent or dependent variable.
- Anomaly
- A value that is outside the general pattern of the results: very high or very low compared with the others.
You will not be asked to recall these definitions word for word, but you must use the words correctly. Precise means the repeats are close to each other. Accurate means close to the true value. Results can be precise but not accurate if there is a systematic error.
Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026