I frequently hear people confuse accuracy and precision.
Problem:
These two nouns are not synonyms, at least not in science.
Explanation:
The scientific definition of the noun “accuracy” is the degree to which a measurement agrees with that measurement’s standard value.
The scientific definition of the noun “precision” is the degree to which a set of measurements of the same sample agree with that set’s mean.
Imagine an archery target, and imagine an archer shooting arrows at that target.
The closer that an arrow lands to the center of the target (the bull’s eye), the more accurate is the shot.
In contrast, if an archer can shoot several arrows that all land in almost the same location, then one can say that the archer is precise.
An archer can be accurate simply by the fact that bull’s eye is the mean of the points at which all of his or her arrows land.
An archer can be precise simply by the fact that all of his or her arrows land in the same location.
Precision speaks to reproducibility and standard deviation within the sample. Higher precision in a trial (e.g., in shooting a set of arrows) refers to greater reproducibility and smaller standard deviation.
Accuracy speaks to nearness to the target. Higher accuracy in a trial (e.g., in shooting a set of arrows) refers to less distance between the target location or value and the mean of the actual locations or values.
This discussion has these implications:
- A trial can be accurate but not precise.
- A trial can be precise but not accurate.
- An ideal trial is both accurate and precise.
- One can make a calculated adjustment for a trial that is precise but not accurate (that is, a trial that is biased), if one knows the degree of inaccuracy (the degree of bias).
- One can ignore a trial’s imprecision (the degree of variability) in a trial that is accurate but not precise, if one’s primary interest is that the mean within the trial is very close to the goal or target.
Solution:
Think “nearness to a goal” when thinking of accuracy. Think “reproducibility” when thinking of precision.