What is the difference between Accuracy and Precision?

We are often told that we should give accuracte and precise estimates, and many people use these two terms synonymously. This post explains what they each mean and how to use them in the context of Estimates.

Both words are very similar, but ultimately, there’s a difference. Something is “accurate” if it is without mistakes, and something is “precise” if it is exact. For example, “Between 1.70m and 1.90m” is an accurate description of my height, and “1.85m” is precise.

Both words do not necessarily mean the same thing: Estimating that something will be done “in the future” is very accurate, but not very precise. Estimating that something will be done on the 5th of August 2019 at 08:43:51 is very precise, but not very accurate (as it will most likely not be done in that exact second).

The biggest risk to be aware of is that people perceive precision as accuracy. When someone tells you something will cost 1,051.43 EUR, you are more likely to assume that this will be very close to the actual price than if the same person told you it would cost 1,000-1,200 EUR, without questioning the basis of your estimate. Just because it’s more precise, it’s somehow more trustworthy.

This is a frequent contributor to wrong estimates, as usually, the context of an estimate gets lost while it is being worked into project plans, e-mails and memos. And depending of who you talk to, it might even get lost mid-sentence, as many people tune out after the “but” in “I estimate it would take 8 hours, but…”!

As a rule of thumb, the precision of your estimate should reflect how comfortable you are with it. Less precise estimates are to be expected the less you know about the topic at hand, and while “between 10 and 150 hours” is dramatically less precise than just “80 hours” (which is the average of both numbers), it reflects the uncertainty of the estimate much better and is ultimately more accurate.