Accuracy vs precision in measurement
Precision is about scatter; accuracy is about closeness to the true value. An instrument can be precise and wrong, or accurate on average and noisy.
A sensor that reads 24.9, 24.9, 25.0, 24.9 °C when the true value is 21.0 °C is precise and inaccurate. One that reads 20.6, 21.5, 20.8, 21.2 °C is imprecise but, on average, close. The first can be fixed by a correction from calibration; the second only by a better sensor or by averaging.
How the concepts map to a budget
- Random effects (precision) enter as Type A repeatability.
- Systematic effects are corrected where known; the uncertainty of the correction enters as Type B.
- Uncorrected systematic effects are treated as uncertainty components, not ignored.
Marketing copy uses 'accuracy' for a specification limit. Metrology uses uncertainty. When comparing instruments, ask what the number is and under which conditions it holds.
Frequently asked questions
- Does high resolution mean high accuracy?
- No. A 0.001 °C display on a sensor with 0.3 °C systematic error is precise-looking and inaccurate.
- Is repeatability the same as precision?
- Repeatability is precision under a specific, tightly controlled set of conditions; reproducibility is precision under changed conditions.
References
- [1]JCGM 200:2012 — International vocabulary of metrology (VIM), 3rd edition
- [2]JCGM 100:2008 — Guide to the expression of uncertainty in measurement (GUM)
General technical guidance written against the cited sources. It is not regulatory or legal advice and does not replace the applicable standard, guideline or a qualified reviewer's judgement.
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Combine Type A and Type B components per the GUM: divisors by distribution, sensitivity coefficients, u_c, effective degrees of freedom and U at k = 2.
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