Accuracy vs Precision in Measurement
Accuracy
How close a measurement is to the true (accepted) value. Accurate results cluster around the bullseye — they are free of systematic bias.
Unit: unit of the quantity
Example: A scale reading 70.1 kg for a true 70.0 kg mass is accurate to 0.1 kg — even if repeated readings scatter a little.
Precision
How repeatable measurements are — how tightly repeated readings cluster together, regardless of where the cluster sits. High precision means low random scatter.
Unit: unit of the quantity
Example: A stopwatch that always reads 9.81 s, 9.82 s, 9.81 s is precise — but if the true time was 10.50 s, it is precisely wrong.
Key differences
| Aspect | Accuracy | Precision |
|---|---|---|
| Answers | 'How close to the truth?' | 'How repeatable?' |
| Target analogy | Shots centered on the bullseye | Shots tightly grouped (anywhere) |
| Ruined by | Systematic error (bias) | Random error (scatter) |
| Can you have it alone? | Yes — accurate but scattered (unbiased, noisy) | Yes — precise but biased (tight cluster off-target) |
| Improved by | Calibration, better technique | More trials, averaging, finer instruments |
A digital display with many digits looks 'accurate' but digits only show precision. A bathroom scale reading 70.135 kg is precise-looking yet can be kilograms off if never calibrated — resolution is not correctness.