Uncertainty & measurement
Instrument readings & zero error
Correct zero offsets and combine reading uncertainties.
Your measurements
Number formats & units
Use decimal dots in datasets. Single values also accept decimal commas and scientific notation such as 1.2e-3 or 1.2 × 10⁻³.
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Result & working
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Interpretation & assumptions
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Formula & example
The relationship
What the variables mean
| Symbol | Meaning |
|---|---|
| Zero offset | What the instrument reads when it should read zero. For a two-reading difference, enter the net offset (often zero). |
| Δreading | Half/full resolution or the supplied absolute uncertainty. |
| Uncertainty in the applied correction, zero if treated as exact. |
Worked example
- Observed reading
- 12.4
- Signed zero offset
- 0.2
- Smallest scale division / display increment
- 0.1
- Reading uncertainty convention
- Half a division per reading
- Number of readings in the measurement
- One reading
- Absolute uncertainty in zero correction
- 0.05
- Reading unit
- mm
(12.2 ± 0.1) mm
Follow the example steps
- Corrected value = 12.4 − (0.2) = 12.2 mm.
- Reading uncertainty under the selected half convention = 0.05 mm.
- Absolute uncertainties add: 1 × 0.05 + 0.05 = 0.1 mm.
- Rounded result: (12.2 ± 0.1) mm.
Common mistakes
Subtract the signed zero offset.
Incorrect approach
5.0 − 0.2 = 4.8
Correct approach
5.0 − (−0.2) = 5.2
More checks before you submit
- Do not confuse resolution, precision and accuracy.
- Subtract the signed zero error, not its magnitude.
- An offset correction does not eliminate uncertainty in the correction itself.
Method notes
A positive zero offset is subtracted; a negative offset is also subtracted and therefore increases the corrected reading. For a length obtained by subtracting two scale readings, enter the observed difference and choose two readings.
Half a division is a common starting convention for an analogue scale. A digital display may be assigned one last digit, half a step, or a manufacturer specification. Follow the actual task; neither choice is a universal IB requirement.
Two readings can share a calibration offset that cancels in their difference. The displayed addition rule is a conservative convention for the uncertainty components you supply; do not count a cancelled offset again.