What is accuracy vs. precision?

• A. The accuracy: closeness to true value; Precision: consistency of measurements.
• B. The accuracy: closeness to true value; Precision: accuracy of the measurement process.
• C. The accuracy: the precision of the instrument; Precision: accuracy of measurement.
• D. The accuracy: how representative measurements are of the true value; Precision: consistency of measurements.

Answer: (D)

Accuracy is about how well measurements reflect the true value on average—the measurements should be representative of the true value, indicating bias is low. Precision is about repeatability—the measurements are consistently close to each other, showing little random variation. The best description thus matches accuracy as how representative the measurements are of the true value and precision as the consistency of measurements.

Think of it this way: if measurements cluster tightly but are all off from the true value, you’re precise but not accurate. If measurements hover near the true value but are spread out, you’re accurate but not precise. If they’re both close to the true value and tightly clustered, you have both accuracy and precision.

The other formulations mix up these ideas: one suggests precision is about the overall accuracy of the measurement process, another implies accuracy is about the instrument’s precision, and another characterizes accuracy as the closeness of individual measurements rather than the representativeness of the set as a whole.