Accuracy vs. Precision: What's the Difference?

What do we mean when we say that results of experiments in science and engineering are precise or accurate? How are precision and accuracy different? That is the subject of discussion in this write-up. For every student of the exact sciences, the study of measurement and an understanding of these two concepts is essential.

The comparison presented in the following lines will help clarify these concepts for you. In colloquial usage, accuracy and precision have the same meaning. However, these two words stand for different concepts, when it comes to science experiments or engineering methods.

Accuracy defines how close a measurement or experimental result is to its true value. It is related to the degree of deviation of a result, from its true value. More closer a measurement is to the correct value, more accurate it is.

Precision is the reproducibility of similar results in an experiment or measurement. It doesn't consider whether the results are close to the true value. As long as the results are consistently or repeatedly close to each other, there is precision in measurement.

So, results of any experiment could be accurate but not precise (close to true value but not repeated or reproducible), accurate and precise (close to true value and reproducible), not accurate but precise (away from true value but reproducible or repeated), or neither accurate, not precise (away from true value and non-reproducible).

The last case is the worst one to have for any experimenter or engineer, when it comes to instruments. The four combinations say a lot about the kinds of scientific experiment results, that may occur, and the way in which accuracy and precision are different.

Accuracy concerns itself with how close you can get to the true value with an instrument, while precision talks about how many times an instrument can come up with a measurement reproducing the same result, or a value close to earlier result.

Precision and accuracy are both equally important. The results of a measurement need to be accurate and reproducible, so that one can draw some conclusion from it. Accurate but non-reproducible results cannot be trusted in an experiment.

Consider a circular target with a bull's eye in the center, at a shooting range. Some shooters have already practiced there and their bullets have left a mark on the wheel. You can judge how good a shooter is, by seeing how accurate and precise his or her shots are.

If you notice that all the bullet marks are scattered all over the place with none close to each other and none close to the bull's eye either, the shooter is neither precise, nor accurate. If you notice that bullet marks are right on the bull's eye region and all clumped together, then you are looking at an accurate and precise shooter.

Hope this comparison has left no doubt in your mind about how these two concepts are different in the context of scientific method of observation and experiment. It is due to the precision and accuracy with which measurements are made, that the manufacturing and working of any machine is made possible.