# Precision Vs Accuracy: Which Is Which and How Can You Remember?

Precision and accuracy are terms used in science to describe the quality of measurements. They mean different things, and sometimes it is difficult to remember which is which. What do they mean and how can you remember which is which.

As an example, let’s work with the length of an object. So, to measure an object’s length you get a ruler. Line up the object you need to measure with the ruler. Measure it. Move the ruler and the object and re-measure your object. Repeat this procedure a few times.

What numbers did you get? Let’s pretend I measured 30mm, 31mm, 30mm, and 30mm for my object.

Precision is something I can control. Precision is my ability to use my tool (the ruler) to get the same measurement for the same object in the same conditions each time. In the example above, I predominantly measured 30mm. Again, precision is related to my skill at using my measurement tool. Precision can be learned. Precision is under your control.

Accuracy, on the other hand, is not under my control nearly as much. Say I take my same object and measure it with a different rule. The new ruler is a different brand and made out of a different material. (Say it’s wood instead of clear plastic.) One of the rulers is 35 years old and one of the rulers is one year old. If I re-measure my object with this different ruler and get 31mm, 31mm, 31mm, and 31.5mm, I’m still getting pretty good precision. My measurements are very close together and I can repeat them. However, my numbers come out differently with this ruler. One of the rulers is more accurate than the other. Which one is it?

Accuracy is how close I can come to the real measurement using the measurement tool I have.

Since 1983 a meter has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second. From 1960 to 1983, however, a meter had a different definition. Back then, the definition of a meter was related to the distance of a certain number of wavelengths emitted from electrons transitioning from one orbital to another in Kyrpton (isotope 86) atoms.

Whichever ruler is closer to the distance light travels in a vacuum in 1/299,792,458 of a second is more accurate.

How can you remember which is which?

I don’t know if this will help you, but I say to myself “Perfectly Precise Polly Abdicated Accuracy.”