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Precision, accuracy, resolution, sensitivity and validity

Edexcel IAL Physics · WPH16 Practical Skills in Physics II

Unit 6 explicitly tests the language of measurement. Each word has a precise meaning, and examiners do not accept one in place of another.

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The definitions

Pearson's practical appendix, and the Unit 6 specification, use these terms precisely:

Term	Meaning
Accuracy	How close a measurement is to the true value. Affected by random and systematic errors.
Precision	How close repeated measurements are to each other. Affected only by random errors.
Repeatability	Precision when the same person repeats the same method with the same apparatus.
Reproducibility	Precision when different people, methods or apparatus are used: a harder test.
Resolution	The smallest measuring interval an instrument can show.
Sensitivity	The change in an instrument's output per unit change in the quantity being measured.
Validity	Whether a measurement or experiment actually measures what it is meant to.
Uncertainty	The interval in which the true value is expected to lie.

Sensitivity in practice

A sensor or circuit is more sensitive if a small change in the input gives a large change in the output. For a thermistor potential divider, sensitivity = change in output p.d. / change in temperature (in V/K or mV/K). On a calibration curve, the sensitivity at a point is the gradient there, and it is usually greatest where the curve is steepest. A sensitive instrument is not necessarily accurate: it can respond strongly and still be wrongly calibrated.

Using the words correctly

Justify an instrument by its resolution, not by saying it is 'more accurate' or 'more precise'.
Results scattered tightly but offset from the true value are precise but not accurate.
A result whose limits include the accepted value is accurate.
A method is valid if it controls the variables that matter and measures the intended quantity.

Worked example

A thermistor circuit's output rises from 2.10 V at 30 °C to 2.58 V at 40 °C. Find the average sensitivity over this range.

Sensitivity = change in output / change in input
= (2.58 - 2.10) / (40 - 30) = 0.048 V/K

Answer: 48 mV/K

Key idea: Accuracy: close to the true value. Precision: repeats close together. Resolution: smallest interval. Sensitivity: output change per unit input change (a gradient). Validity: measures what it should.

Check yourself

A sensor's output changes by 0.84 V when the temperature rises by 20 K. Find its sensitivity in mV/K.
Show answer
42 mV/K. 0.84 / 20 = 0.042 V/K = 42 mV/K.
Which statement defines precision?
- How close repeated measurements are to each other
- How close a measurement is to the true value
- The smallest interval an instrument can show
- The change in output per unit change in input
Show answer
How close repeated measurements are to each other. Precision is about the spread of repeats.
A student's value of g has limits 9.70 m/s² to 9.90 m/s². The accepted value is 9.81 m/s². Which word describes the result?
- Sensitive
- Reproducible
- Accurate
- Valid only
Show answer
Accurate. The accepted value lies within the limits, so the result is accurate.
On the calibration curve for a thermistor circuit, where is the circuit most sensitive?
- Where the curve is flattest
- Where the curve is steepest
- At the lowest temperature always
- Where the output p.d. is largest
Show answer
Where the curve is steepest. Sensitivity is the gradient: the steepest part gives the largest output change per kelvin.
Several different students using different apparatus all get similar values. What does this show?
- The result is repeatable only
- The instruments have high resolution
- The result is sensitive
- The result is reproducible
Show answer
The result is reproducible. Agreement across different operators and apparatus is reproducibility.
A student justifies using digital calipers by saying 'they are more precise'. What would examiners prefer?
- They are more accurate
- Their resolution is 0.01 mm, so the percentage uncertainty is small
- They are more sensitive
- They are digital
Show answer
Their resolution is 0.01 mm, so the percentage uncertainty is small. Examiners do not accept precision or accuracy as substitutes for resolution.
A light sensor's output rises from 1.25 V to 1.85 V when the illumination increases from 200 lux to 500 lux. Find its average sensitivity in mV per lux.
Show answer
2.0 mV per lux. 0.60 V / 300 lux = 0.0020 V/lux = 2.0 mV per lux.

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Last updated 8 November 2026. Spotted a mistake? Tell us.