Precision Versus Accuracy – Definitions and Differences

accuracy-vs-precision

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

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.

For Example:

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

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.

For Example:

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.

Comments

  1. Sam says

    Good examples and explanation, but can you please elaborate the below:

    in your first example where results are 10.490, 10.495, 10.500…. can we say it is both accurate and precise as result is close to 10.

    Similarly in the second example, 9.8, 9.9 and so on… can we still say it is precise and accurate??? as again values are less scattered (in the given sample) and close to 10?

    Also, i will appreciate if you can clarify another question. (based on Oliver Lehmann’s questions)… Qs.9 from 175 questions http://www.oliverlehmann.com/contents/free-downloads/175_PMP_Sample_Questions.pdf

    the options are “process should be adjusted” or improved… the correct answer goes with adjusted…

    i want to know if there is any rule of when to adjust or improve? this is not clear at all and i can’t find any good reference :(

    • Fahad Usmani says

      In my first example, all values are close to 10.5 meters means they are precise (note that, required length is 10.00 m). Though they are not close to the target length.

      In second example, values are moving around 10.0 meters, meaning they are close to the target length but not very precise (they are not very close to each other).

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