by Leon Rosenshein

Precision vs. Accuracy

Everyone wants to be accurate, and everyone wants to be precise. Those are both great goals. It’s a wonderful thing when you can be precise and accurate. On the other hand it becomes a problem when you trade one for another or even worse, mistake one for the other.

What’s the difference between precision and accuracy? The way I think about it, precision is a measure of the “size” of a quanta. One hour has 1/60th precision of a minute, and a year is 1/525600 the precision. Precision is a measurement of the measurement you’re making and has nothing to do with the thing being measured. If you measure in whole years there was a 1 year time period (525600 minutes) when I was 21 years old. If you were to measure in whole hours there was a 1 hour time period (60 minutes) when my age was 184086 hours. The measurement of 184086 hours old is much more precise than 21 years old. Measure it in minutes or seconds and it’s still more precise.

Accuracy, on the other hand is a measurement of how close the measurement is to truth, however you want to define truth. Going back to the age example, if I were to tell you I was 54 years old I would be 100% accurate. However if I told you I was 473364 hours old I would be almost 2% off. Both 54 years and 473364 hours represent the same timespan, but the accuracy of the two is different.

Of course the two are intimately related. Consider the birthday example. My birth certificate is considered truth, and it has a time of birth with a precision of 1 minute. But what’s the accuracy? We don’t really know. How much time passed between being born and looking at the clock? Probably not much, but some. And how precise/accurate was the clock? When was it last set? To what standard? It was almost certainly an analog clock, so the angle of view can change the reading as well. In my case it doesn’t make much difference, but consider the person with a birth time of 2359 local. That’s one minute of precision, but an accuracy slip of 2 minutes has the person born on the wrong day. And if it was December 31st it could be the wrong year as well.

Ballistics is another area where the difference between the two is apparent. Back in my earlier days we talked about Circular Error Probable (CEP) a lot. For a given initial condition how tight of a cluster would a bomb land in. How big would a circle need to be to include 90% (CEP-90) of the attempts? The smaller the circle, the better, and more precise, the system was. But that doesn’t say anything about accuracy. The bombsight could have been anywhere, but the CEP would be the same. Getting the bombsight and the center of the CEP to match is the accuracy. That was my job, and that sticking actuator gave me a lot of grief before I had enough data and didn’t have to worry about the size of the CEP, but that’s another story.

As engineers we know all this. We’ve been taught about it and have dealt with significant figures in math for years. But what does this have to do with development? It’s important when it comes to making estimates. Ideally you can be both accurate and precise, but that’s both hard and rare. In that case I say accuracy is more important. And even more important is to not confuse the two. Just because we estimate in hours or sprints, doesn’t mean we know to the nearest hour when something will be done. We need to be careful to not conflate the precision of the anwer with it’s accuracy. It’s an estimate. And it will get more accurate as time goes on and we know more and get closer to the end. But it rarely gets more precise. How to deal with that is a topic for another day.