What is the difference between **PRECISION** and **ACCURACY**? Upon first glance, many students would say that the two terms mean the same thing. In normal day-to-day usage, you would talk about how precise this is, or how accurate that is, and in both cases you would be comparing “how close” some thing is to the actual thing.

In Math and Science, however, they are slightly different.

*ACCURACY *has the meaning that I just described: how close a number is to the actual number.

*PRECISION*, on the other hand, does not refer to how close a number is to an actual number, but rather refers to how many digits and decimal places a number has. (A separate definition for precision also is that it measures how close repeated measurements are to each other, rather than to the actual value.)

As an example, consider a 100 gram block of wood. We are told that this block has a mass of 100 grams. Now, if we put it on balance A, we get a mass of 99.9 grams. When we put it on balance B, we find a mass of 101.350022. We can say that balance A provides us with a more ACCURATE number because it is closer to the known mass of 100 g, but balance B gives us a much more PRECISE number because it displays more decimal places. However, even with the extra decimal places, it isn’t as accurate as balance A. (Maybe there is something wrong with its calibration?)

Do you think that some measurement can be both accurate AND precise? Sure it can.

Let’s suppose we have a bottle of pop. The label says that it is a 1000 mL container. We pour the pop into a graduated cylinder, and determine that there is 998 mL. Since this particular graduated cylinder that we used has no decimal markings, the whole number 998 is as precise as we can get. But that is as precise as the label says. We measure only a 2 mL difference, which isn’t that much (0.2%). So, with this cylinder, we can say that we have a fairly accurate and precise measurement.

If we could determine that the volume in the container was 998.93188 mL, we could say that this number was accurate, and very precise.

Hopefully these examples were able to show you the difference between accuracy and precision. Please comment if you would like some further explanation; this concept always confuses students. I will do my best to clarify.