Precision versus Accuracy is an important concept in the quality management knowledge area. And to understand various Quality Management concepts, it is necessary for you to have a thorough understanding (definition and difference) about commonly used terms in Quality Management, which are often misunderstood as synonyms to each other; e.g. Precision vs Accuracy, and Quality vs Grade.
In this blog post I’m going to discuss the definitions and differences between Precision and Accuracy. In the next blog post we will talk about Quality versus Grade.
Okay, let’s get started…
Precision means the values of repeated measurements are clustered and have little scatter.
Precision doesn’t mean that the measurements are close to the target value – it means that the measurements are close to one another. They may or may not be near the target value.
Precision is about how the measured values are close to one another. If the scatter is lesser, measurements are said to have a high precision.
Let’s say that you are manufacturing rods ten meters in length. During the manufacturing process, for quality control purposes you take the measurements of five rods.
The length of each rod is as follows:
- First Rod: 10.490 meters
- Second Rod: 10.495 meters
- Third Rod: 10.500 meters
- Fourth Rod: 10.505 meters
- Fifth Rod: 10.510 meters
If you analyze these measurements, you will notice that although these measurements are not close to the target value of ten meters, but they are very close to one another. There is very little difference in the measurements among them, or we can say that scatter is very small.
In this case we will say that the measurements are very precise.
Precision is a measure of variation among the measured values.
Accuracy means the measured values are very close to the true value.
If somebody says that measurements are accurate, then you should know that those measurements are very near the target, or true value. Scatter doesn’t have any significant role here. The scatter of accurate measurements may, or may not be dense.
Let us take the same example again. This time you find the measured length of each rod as follows:
- First Rod: 9.900 meters
- Second Rod: 9.800 meters
- Third Rod: 10.000 meters
- Fourth Rod: 10.100 meters
- Fifth Rod: 10.200 meters
You can see that all measurements are very close to the target length of the Rod. They are all near the required length of ten meters. You can also see that; though, these measurements are close to the target value but there is a big variation among them; i.e. higher scatter.
Here, the measurements are very accurate but not very precise.
It is not necessary for precise measurements to be accurate, or accurate measurements to be precise.
It is the responsibility of the project management team to decide the level of the accuracy and precision for their project.
This was all about precision versus accuracy; I hope I made the points clear to you.
Image credit => NOAA’s National Ocean Service.