OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(d) precision and accuracy of measurements and data, including margins of…
Full specification wording
Evaluation: (d) precision and accuracy of measurements and data, including margins of error, percentage errors and uncertainties in apparatus
Precision describes how closely repeated measurements agree with each other, while accuracy describes how close a measurement is to the true value.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 2 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Precision describes how closely repeated measurements agree with each other, while accuracy describes how close a measurement is to the true value. A precise set can still be inaccurate if a systematic error shifts all values. Uncertainty in apparatus is often taken as half the smallest division for an analogue scale, or the stated resolution for a digital display. The margin of error is the range within which the true value is expected, such as a reading of 25.0 cm³ ± 0.1 cm³. Percentage error compares the uncertainty with the measured value: for a 50.0 cm³ reading with uncertainty 0.1 cm³, the percentage error is (0.1 ÷ 50.0) × 100% = 0.2%. Combining uncertainties in a calculation follows the rules for adding, multiplying or raising to a power.
What earns the marks
- Distinguishes precision, the agreement between repeats, from accuracy, the closeness to the true value.
- States that a precise set of readings can be inaccurate if a systematic error is present.
- Determines the uncertainty in an apparatus reading, for example half the smallest division for an analogue scale.
- Expresses a measurement with a margin of error, such as 25.0 cm³ ± 0.1 cm³.
- Calculates percentage error as (uncertainty ÷ measured value) × 100%.
- Combines uncertainties correctly for addition, multiplication or powers.
Common mistakes and how to put them right
Treating precision and accuracy as the same thing.
Put it right: Precision is about repeatability, accuracy is about closeness to the true value.
Forgetting to convert an uncertainty into a percentage before combining it with other percentage uncertainties.
Put it right: Convert each to a percentage, then add for multiplication or division.
Using the full smallest division as the uncertainty for an analogue scale.
Put it right: Use half the smallest division unless the apparatus states otherwise.
Exam tips
- Write the uncertainty with the same number of decimal places as the reading.
- Show the percentage error calculation clearly, including the division and the multiplication by 100%.
- Check whether uncertainties should be added or combined in quadrature before quoting a final value.
Try the lesson
The start of the Evaluation: (d) precision and accuracy of measurements and data, including margins of error, percentage errors and uncertainties in apparatus lesson, made from checked teaching notes for this specification point.
Listen
Precision describes how closely repeated measurements agree with each other, while accuracy describes how close a measurement is to the true value. A precise set can still be inaccurate if a systematic error shifts all values. Uncertainty in apparatus is often taken as half the smallest division for an analogue scale, or the stated resolution for a digital display. The margin of error is the range within which the true value is expected, such as a reading of 25.0 cm³ ± 0.1 cm³. Percentage error compares the uncertainty with the measured value: for a 50.0 cm³ reading with uncertainty 0.1 cm³, the percentage error is (0.1 ÷ 50.0) × 100% = 0.2%. Combining uncertainties in a calculation follows the rules for adding, multiplying or raising to a power. A strong answer earns marks for these points. Distinguishes precision, the agreement between repeats, from accuracy, the closeness to the true value. States that a precise set of readings can be inaccurate if a systematic error is present. Determines the uncertainty in an apparatus reading, for example half the smallest division for an analogue scale. Expresses a measurement with a margin of error, such as 25.0 cm³ ± 0.1 cm³. A common slip: Treating precision and accuracy as the same thing. Instead, precision is about repeatability, accuracy is about closeness to the true value. Exam tip: Write the uncertainty with the same number of decimal places as the reading.
Key words
Say what each word means before you turn the card.
Precision
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Accuracy
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Margin of error
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Percentage error
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See every definition
- Precision
- Describes how closely repeated measurements agree with each other.
- Accuracy
- Describes how close a measurement is to the true value.
- Margin of error
- The range within which the true value is expected, such as a reading of 25.0 cm³ ± 0.1 cm³.
- Percentage error
- Compares the uncertainty with the measured value, calculated as (uncertainty ÷ measured value) × 100%.
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