Last updated on January 24, 2016
Accuracy and Precision
Do you know there is a difference between Accuracy and Precision?
Hmm…Really? But why bother? On the face of it, these terms look to be the same.
Well! You should bother if you are a Quality professional or a Project Manager. And if you are a PMP® aspirant then it definitely warrants a clear understanding. It may help you in solving one or two questions correctly.
On the face of it, the meaning of Accuracy and Precision seems to be the same. But, there is a small but important difference between the two.
Let us understand the difference between Precision and Accuracy with the help of a small example.
Accuracy means that the measured value is very close to true value (target or goal). Accuracy is about correctness.
Precision means the values of repeated measurements are clustered and have a little scatter. Precision is about exactness.
Precision and Accuracy are not equivalent. Precise measurement are not necessary accurate. An accurate measurement is not necessarily precise.
These definitions are precisely accurate. But they are confusing. Let us go beyond the definitions.
Refer to the adjoining figure. Let us assume that it represents a dart-board. The target or goal (true value) is to put darts exactly in the center (Bull’s Eye) of the dart-board. The blue circle in the adjoining figure represents our goal.
Let us assume that we hit 4 darts and all of them landed inside the blue circle. We can say that we met our our goal. Or in other words we can say we were Accurate (Correct).
Let us assume that we hit another 4 darts and all of them landed in the grey circle. The darts landed just outside the blue circle. All the 4 darts were very close to each other. We did not meet our goal but there was hardly any space between the darts. We can say that there was very little scatter. Or in other words we can say we were Precise (Exact).
Blue circle darts are Accurate but not Precise (too much of Scatter). Grey circle darts are Precise but not Accurate (not hitting the goal).
an Accurate measurement may not be Precise
a Precise measurement may not be accurate.
A project manager along with other stakeholders (who are providing the requirements) must determine required level of accuracy and precision for measurements. Both are important for the project.