Don’t Confuse Accuracy and Precision (in Microohmmeters and More)

Two terms often bandied about in the test and measurement industry are accuracy and precision. Sometimes these two terms are used interchangeably, but they really mean different things.

The New Oxford American Dictionary gives the technical definition of accuracy as “the degree to which the result of a measurement, calculation, or specification conforms to the correct value or a standard.” When we say an instrument is very accurate, we mean that the measurements that we make with it will be very close to the actual value of the parameter we are measuring.

The dictionary also gives a more colloquial definition for accuracy, “the quality or state of being correct or precise.” This definition equates accuracy and precision, but in test and measurement, the definition is quite different. The technical definition for precision is “the degree to which a number of measured values agree.” In this sense, precision is more akin to repeatability than to accuracy.

Let’s look at an example. Figure 1 shows the bullseyes from four rounds of target practice. Shooter #1 was both accurate and precise. All of his shots hit the bullseye. Shooter #2 was precise, but not accurate. His shots were all close to one another, but he missed the bullseye by a wide margin. Shooter #2 was accurate but not precise. His shots were close to the bullseye, but scattered. Shooter #4 was neither accurate nor precise. His shots were all over the place.

In a similar way, measurements can be accurate, but not precise. Figure 2 shows the results of 1,000 measurements of a 200 mΩ resistors. The graph on the left shows the measurements made with our 1750 High-Speed Microohmmeter. The graph on the right shows the measurements made with a Keithley 3706 Microohmeter. Both microohmmeters meet their accuracy specification, but as you can see, the 1750 is much more precise.

 
When an instrument is precise as well as accurate, you can be more confident in the measurements that you make. And, in a production environment, this translates into less time spent making measurements and and more time spent increasing production yields.

Don’t confuse accuracy and precision. They are not the same thing, and in many applications, precision is just as important, and sometimes more important, than accuracy.

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