Are accuracy, precision and repeatability the same thing?

Learn with ABB | 26 April 2023

The ability of analytical instruments to provide consistent, reliable and accurate measurement is crucial to helping ensure they provide the expected levels of performance. Over the coming weeks, we will be publishing a series of blogs covering the various key issues around determining, quantifying and maintaining the accuracy of analytical equipment.

In this first blog of the series, we look at three terms that you will often see when specifying an analytical instrument – accuracy, repeatability and precision.

Don’t be fooled…

To the uninitiated, the terms accuracy, repeatability and precision might appear to all be pretty mutually exclusive. However, in the world of instrumentation, there is actually a distinct difference between the terms, making it important to understand what each one means and its relation to the others.

Let’s start by looking at their definitions.

Accuracy

The accuracy of instrumentation is determined by the difference of a measured value compared to its actual (true) value. As no measurement is 100% exact an element of inaccuracy needs to be considered, hence the reason why accuracy figures are quoted with ‘±’. Ultimately, accuracy measures how close you come to the correct result. Your accuracy improves when your instruments or tools are calibrated properly.

Precision

Precision is the repeated measurement of a set of values relative to each other, rather than the actual value. Precision improves when using finely incremented tools that require less estimation; better equipment and improved procedures equals better precision.

Repeatability

Repeatability allows you to measure how close a particular result or set of data is compared to the same measurement, made with the same device or instrument, under the exact same circumstances. In other words, the measurement procedure, observer, device or instrument, testing conditions and location would all need to be exactly the same and testing needs to be conducted over a short space of time.

Illustrating the difference

Getting to grips with these definitions and how they affect each other can be quite complicated. To make things easier to understand, consider the simple diagrams below.

Imagine you’re at an archery range and have a bow with a telescopic sight. You take aim and shoot. After you finish and take a look at the target, the arrows are sprayed all over the place.

You decide to alter the sight as the grouping isn’t close enough. You add an improved sight, with sharper and better magnification. Does this help?

No, not really.

The cluster of arrows are much closer together (precision) but they are still too far from the bullseye (your desired target), so the accuracy hasn’t improved. By this measure, precision has improved but accuracy hasn’t.

Let’s now switch this around, improving our accuracy but not our precision. Rather than alter the setup for the telescopic sight, we take more care and align the bow properly. As can be seen by the target, accuracy has improved dramatically but the arrows are spread wide apart across the whole target.

It’s an improvement, true, but you’re still not quite there. The set-up of the bow is perfect; you just need to add the sharper, more magnified sight. Once this is done, you shoot – and hey presto! With the correct alignment and optic you have high accuracy and high precision.

Now that the bow and sight are setup correctly and the precision and accuracy have been altered to its correct measurement, we have the ability to carry on shooting arrows at the target and get the same result over and over again (repeatability).

From the above examples, it is clear that measurement accuracy and precision can be independent of each other and that repeatability relies on getting the exact balance time after time.

How can ABB help?

Ensuring that your measuring instruments are reliable is essential. Throughout a device’s operational life, many elements can conspire to reduce the effectiveness of its measuring capabilities, whether through general wear and tear, lost documentation or a lack of technical knowledge.

Failing to perform frequent maintenance checks can be extremely costly in the long-term, so it is vitally important to regularly calibrate the instrument’s accuracy, precision and repeatability.