Precision, repeatability, reproducibility, resolution and accuracy.

The idea

These five terms describe different qualities of measurement and are easily confused. Precision describes how closely repeated readings agree with each other, so a precise set is tightly clustered. Repeatability is the closeness of repeated readings taken by the same person, using the same method and equipment, over a short time; reproducibility is closeness when the measurement is repeated by a different person, or with different equipment or conditions. Resolution is the smallest change an instrument can detect, such as 0.01 mm on a digital micrometer or 1 mm on a metre rule. Accuracy is how close a reading is to the true value. A set can be precise but inaccurate if a systematic error shifts every reading, and an instrument with fine resolution can still give inaccurate results if it is miscalibrated.

What earns the marks

• Defines precision as the closeness of agreement among repeated measurements of the same quantity, indicated by a small spread of values.
• Defines repeatability as agreement between repeats made by the same person, with the same method and equipment, over a short interval.
• Defines reproducibility as agreement between measurements made by a different person, or with different equipment, method or conditions.
• Defines resolution as the smallest change in the measured quantity that produces a detectable change in the instrument reading.
• Defines accuracy as the closeness of a measured value to the true value, and explains that precision and accuracy are independent qualities.
• Uses a context to separate the terms, for example a miscalibrated digital balance giving closely agreeing but inaccurate masses, or a metre rule whose 1 mm resolution limits the precision of a length.

Common mistakes and how to put them right

• Using precision and accuracy interchangeably.

  Put it right: Precision concerns agreement between repeats, whereas accuracy concerns closeness to the true value, so a precise set can be inaccurate.

• Confusing resolution with accuracy.

  Put it right: A fine-resolution instrument can still be inaccurate if it is miscalibrated, because resolution only describes the smallest detectable change.

• Treating repeatability and reproducibility as identical.

  Put it right: Repeatability involves the same person, method and equipment, while reproducibility involves a change of person, equipment, method or conditions.

Exam tips

• Define each term in one sentence before applying it, so the distinction is explicit in your answer.
• When comparing two data sets, comment separately on their spread (precision) and their closeness to the true value (accuracy).
• Quote the resolution of an instrument with its unit, for example 0.01 mm for a digital micrometer, and link it to the uncertainty in a single reading.