Somewhere between college and graduate school, perhaps in that limbo period when I first tried to write seriously (with little success), I developed a definition: “A scientist is a person who knows the difference between accuracy and precision.” To the best of my knowledge, it was an original thought, though I’ve since heard others say it, so it probably wasn’t very original. It came to mind again the other day as I talked with a colleague, and I thought I’d lay it out in print. Some casual definitions of the terms:
Precision – there are two ways to define precision – the degree of detail offered by a measuring tool, or the level of detail at which a measurement can be reproduced. So, if your bathroom scale gives you a reading of 74.254 kg, it’s precise to .001 kg. Alternatively, if two immediately sequential readings on the scale give 74.251 and 74.256, you might say your scale is precise to .01 kg.
Accuracy – the degree to which your measuring tool gives you the correct value. If you know from other sources that you weigh 74.254 kg exactly, but your bathroom scale says 74.353, you might say that your scale is accurate to within 1 kg.
Precision does not require accuracy. For example, assume you know the time to be 10:00 in the morning, on the nose. Your digital watch says the time is 11:32 and 25 seconds. The watch is then precise down to the second. But it’s not very accurate, and is functionally useless as a time teller, despite its precision. Alternatively, let’s say you have a stylish analog watch with just an hour hand and a spot for noon. This watch may be terrifically accurate – when it’s 12:00, the hour hand is exactly lined up on the noon spot. But because it only has one hand, it’s not very precise – it would be difficult to tell the difference between 10:23 and 10:27 quickly or with much certainty.
This all seems pedantic, but it’s an important difference. Not only to scientists and technologists, but to writers. It’s hard to bring people into your world when the jargon’s not convincing. It’s surprising how often people get this wrong; astoundingly, even some dictionaries conflate the two terms.
Science relies on measurement. A lot of science is all about measurement. And it’s hard to do science if you don’t know what your tools are offering you. People who understand the difference between accuracy and precision, regardless of the work they do, have at hand one of the basic principles they need to do real science. Maybe it’s a stretch to call them scientists, but I’d argue they’re at least capable of being scientists. And in fact, most people who understand this distinction also know something about the scientific method.