What mistakes do students make with Measurements and uncertainties?

Using 'accurate' and 'precise' interchangeably. They describe different things: closeness to true value versus closeness of repeats. Assuming precise data must be accurate. A systematic error can make precise data inaccurate. Thinking that accuracy can be improved by taking more readings. Accuracy requires calibration or correction of systematic errors.

What are the exam tips for Measurements and uncertainties in Physics OCR A-Level?

When asked to compare, state whether the data are precise, accurate, both or neither, and justify with reference to spread and true value. Use the terms correctly: precision for repeatability, accuracy for closeness to true value. If a graph shows a straight line with a non-zero intercept, comment on accuracy and suggest a systematic error. Quote uncertainties to support statements about precision.

OCR · A-Level · Physics

Course version: H556 · Version from Sept 2015

Measurements and uncertainties: (b) precision and accuracy

Precision describes how close repeated measurements are to each other, often indicated by the spread or uncertainty.

Checked against the OCR H556 specification (opens in a new tab) on 10 November 2026.

Point 2 of 4
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Precision is the closeness of repeated measurements to each other.
Accuracy is the closeness of a measurement to the true value.
Precise readings can be inaccurate if there is a systematic error.
Accurate readings can be imprecise if there is large random scatter.
Precision is often quantified by the spread or uncertainty of repeated readings.

Common mistakes and how to put them right

Using 'accurate' and 'precise' interchangeably.
Put it right: They describe different things: closeness to true value versus closeness of repeats.
Assuming precise data must be accurate.
Put it right: A systematic error can make precise data inaccurate.
Thinking that accuracy can be improved by taking more readings.
Put it right: Accuracy requires calibration or correction of systematic errors.
Confusing precision with the number of decimal places.
Put it right: Precision relates to the spread of repeated measurements, not just the format.

Exam tips

When asked to compare, state whether the data are precise, accurate, both or neither, and justify with reference to spread and true value.
Use the terms correctly: precision for repeatability, accuracy for closeness to true value.
If a graph shows a straight line with a non-zero intercept, comment on accuracy and suggest a systematic error.
Quote uncertainties to support statements about precision.

Try the lesson

The start of the Measurements and uncertainties: (b) precision and accuracy lesson, made from checked teaching notes for this specification point.

Listen

Precision describes how close repeated measurements are to each other, often indicated by the spread or uncertainty. Accuracy describes how close a measurement is to the true value. A set of readings can be precise but not accurate if they cluster around a wrong value, for example due to a systematic error. Conversely, readings can be accurate on average but imprecise if they are scattered. In experimental work, precision is improved by careful technique and repeated readings, while accuracy is improved by calibration and eliminating systematic errors. When evaluating data, comment on both the spread (precision) and the closeness to the true value (accuracy). A strong answer earns marks for these points. Precision is the closeness of repeated measurements to each other. Accuracy is the closeness of a measurement to the true value. Precise readings can be inaccurate if there is a systematic error. Accurate readings can be imprecise if there is large random scatter. Exam tip: When asked to compare, state whether the data are precise, accurate, both or neither, and justify with reference to spread and true value.

See it

A top-down flowchart starting from a node labelled 'Repeated measurements', which branches into two nodes: 'Precision: closeness of repeats to each other' and 'Accuracy: closeness to the true value'. The precision node leads to 'Indicated by spread or uncertainty', which leads to 'Improved by careful technique and repeated readings'. The precision node also leads to 'Precise but not accurate if clustered around a wrong value'. The accuracy node leads to 'Affected by systematic error', which leads to 'Improved by calibration and eliminating systematic errors'. The accuracy node also leads to 'Accurate on average but imprecise if scattered'. — It shows how precision and accuracy differ, what indicates each, how each is improved, and how readings can be one without the other.

Key words

Say what each word means before you turn the card.

Precision

Try answering aloud or in your head before turning the card.

Accuracy

Try answering aloud or in your head before turning the card.

See every definition

Precision: Describes how close repeated measurements are to each other, often indicated by the spread or uncertainty.
Accuracy: Describes how close a measurement is to the true value.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart starting from a node labelled 'Repeated measurements', which branches into two nodes: 'Precision: closeness of repeats to each other' and 'Accuracy: closeness to the true value'. The precision node leads to 'Indicated by spread or uncertainty', which leads to 'Improved by careful technique and repeated readings'. The precision node also leads to 'Precise but not accurate if clustered around a wrong value'. The accuracy node leads to 'Affected by systematic error', which leads to 'Improved by calibration and eliminating systematic errors'. The accuracy node also leads to 'Accurate on average but imprecise if scattered'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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