1.8: Accuracy and Precision

Scientists make repeated measurements of a quantity during experimentation to ensure that their results are accurate and precise.

The accuracy of a measurement is the degree of closeness of the results to the true or accepted value.

Consider two students, A and B, who repeatedly weighed a gold bar known to have a true mass of 10 grams. Students A and B each reported three values from repeated measurements of the gold bar. Student A reported values closer to the true mass of the bar compared to student B. Thus, measurements by student A were, therefore, more "accurate".

Precision, on the other hand, is the measure of how closely the results agree with each other, or how reproducible they are.

A measurement is said to be precise if it gives highly similar results when repeated under the same conditions.

For instance, the values for the mass of the gold bar reported by student B were very similar to one another, as compared to student A. That is "precision".

Accuracy and precision are two distinct qualities of measurement which are independent of each other. Thus, a particular set of measurements can be either accurate, or precise, or neither, or both.

Highly accurate values tend to be precise too. Like a weighing balance showing true or close to true masses for all of the objects, repeatedly. However, highly precise measurements may not necessarily be accurate - if the same balance is improperly calibrated, it may give precise but inaccurate readings. This may lead to scientific errors.