When we complete a piece of work or an estimate of something, we might say it was very accurate, or very precise. But what is the difference between the two?
In common language, both may be used interchangeably, but in engineering, mathematics or other technical areas, the two have quite different meanings.
Accuracy
Being accurate means being very close to the mark you were aiming to hit. Accuracy measures how close a number or result is to the actual value desired.
For example, if I shoot an arrow at a target, I can measure my accuracy by how close the arrow lands to the bullseye.
In a more technical example, if I am about to shoot a round of explosives, I can measure how accurate I have been with my shot by how close the result came to what was planned or designed.
Precision
Precision basically describes the repeatabilty of your estimate, answer or outcome. It can also be thought of as how close you came to what you intended, even if it was not correct.
To continue our archery example, if my first arrow hits well below the bullseye (not very accurate), but my second arrow hits almost in the exact same position as the first, I am very precise.
Or I might be trying to stick as close as I can to the speed limit on the highway. I'm sitting on exactly 100km/h, very precise. But unfortunately not very accurate, as the speed limit in this section is only 80!!.
To continue our shot-fiting example above, I've let the blast go and it looks exactly how I planned. Unfortunately I got the wrong plan and only blasted half as much as I should have - I've been very precise and could repeat my blast again, but not very accurate to what the correct result should have been.
Summary
It is possible to be accurate but not precise, precise but not accurate, both accurate and precise, or neither. Ideally we are always looking to be both accurate and precise, and there are circumstances where one is more important than the other.
Hope this clears it up!