Precision vs accuracy

Today we will distinguish two quality management terms: precision vs accuracy.

Most professionals misunderstand the difference between these two terms. Precision and accuracy are often assumed to be synonyms of each other, which is incorrect. It’s easy to confuse the two; in fact, I was once unaware of the difference between accuracy and precision. I came to understand the distinction while preparing for the PMP exam.

Since the two concepts are so often confused, I am writing this post so you can also have a better understanding of these quality management terms.

Let’s get started.

Precision Vs Accuracy

Precision and accuracy are characteristics of measurement. These characteristics collectively determine if the deliverable is meeting project requirements. If the deliverable’s characteristics are not accurate and precise, we would say it’s poor quality.

As a project manager, you analyze the dimensions of deliverables, and if the measurements are accurate and precise, you will accept them. If not, you will ask for corrective action.

An acceptable deliverable must be both accurate and precise.

Precision

Measurements are precise when the values of repeated measurements are clustered close together and have minimal scatter. In other words, the discrepancy between the values is very small.

Precise measurements are not necessarily close to the target value; it just means that the results are close to one another. These measurements may or may not be near the target value.

Measurements are said to have high precision when the scatter is minimal.

An Example of Precision

Assume that you received an order to supply 10,000 rods, 10 meters in length, to your client. You have started production, and during the quality inspection, you randomly measure five rods.

The length of each rod is as follows:

precision example

If you analyze these measurements, you will notice that they are not close to the target (10 meters) but are very close to one another. There is very little difference in their lengths, so their measurements have very little scatter or deviation.

In this case, the measurements are precise.

Precision is a measure of the variation among values.

Accuracy

Accuracy is defined as how close the measured values are to the target value.

Scatter doesn’t have a significant role here. Accurate measurements may or may not be close to one another. 

In other words, accurate data does not have to be precise, but it is ideal.

An Example of Accuracy

Let us consider the example discussed earlier. From a production lot, you randomly pick five rods for a quality inspection. You measure their lengths, and the dimensions are as follows:

accuracy example

You can see that all measurements are very close to the target length of the rod, 10 meters. Although the values are closer to the target value, they are not close to each other, and the scatter is high.

So, you can say that these measurements are accurate but not precise.

You may be wondering which characteristic of measurement is more desirable.

The answer is “accuracy.”

This is because all data are close to the actual value, which is a sign of the correctness of a deliverable. However, the ideal scenario is when the measurements are precise as well because they are both close to the target value and very close to each other.

The Difference Between Precision and Accuracy

accuracy-vs-precision
  • Accurate data are close to the target value, while precise data are close to each other.
  • Accuracy is always desired, while precision is desirable when it is coupled with accuracy.
  • Accurate data can be precise, while precise data may or may not be accurate. 
  • Precision and accuracy are independent of each other.
  • One measurement is enough for accuracy, while precision requires many measurements.

The Significance of Precision and Accuracy

Measurements are vital for quality management. If the measurements are precise as well as accurate, you can say that the product is defect-free.

However, if the measured data is neither precise nor accurate, the product is defective; i.e., it lacks correctness and exactness at the same time, and you have to take corrective and preventive action.

Summary

Precision and accuracy are vital quality management concepts. The key distinctions: accuracy is about closeness to the required value, while precision measures repeatability. Precision alone is not important unless it is coupled with accuracy. Precise measurements don’t need to be accurate or accurate measurements to be precise. Precise measurements can be accurate or inaccurate, and accurate measurements can be precise or imprecise. 

It is the responsibility of the project management team to decide the level of accuracy and precision for their project deliverables during the quality inspection.

These topics are important from a PMP exam point of view, so it’s necessary to understand the key differences between these concepts thoroughly.  

Precision vs accuracy is sometimes a confusing concept, so if you have any thoughts or feedback, please share them through the comments section below.

Image credit => NOAA’s National Ocean Service