accuracy vs. precision

[Jack8rkin]

Hello!
Could you please explain the difference in the meanings of "precision" and "accuracy"?
When should I use "accuracy", and when "precision"?
Thanks

[SoothingDave]

Not a teacher. Am an engineer.

"Accuracy" is the ability to make exactly what it is you are intending to make. If you think of shooting a series of arrows at a target, accuracy is when you hit the center of the target. Accuracy is your ability to hit what you are aiming for.

"Precision" is being able to do the same thing over and over. It is the "grouping" of the arrows you shoot, how close they are to hitting the same spot over and over.

Precision is your ability to repeat what you are doing with little variation. Accuracy is the ability to hit your target.

[euncu]

***neither a teacher nor a native-speaker***

If you are to measure the diameter of a cylinder (let's say that it has a diameter of 50mm [Ø50]), and the measurement is asked to be accurate, than you should measure it as 50 mm, neither 49 mm nor 51 mm is excepted.

If the measurement needs to be precise, than you should measure it as something like this;

Ø50....0...........= 50,000 mm
........-0,016.....= 49,984 mm

[SoothingDave]

Yes, in terms of measurement, accuracy is again the target and precision is the ability to repeatably measure the same thing over and over. How many decimal places are significant.

You can specify a 50.000 mm radius, but if you can only measure to 0.1 mm with any precision, then you need a better (more precise) measuring tool to guarantee your accuracy.

[Jack8rkin]

Thanks a lot!
You hit the target. It's bull's eye!
I mean, the engineer's opinion is invaluable to me, because I translate for engineers.

[bertietheblue]

There's a whole wikipedia article about the differences in meaning:

http://en.wikipedia.org/wiki/Accuracy_and_precision#Accuracy_versus_precision.3 B_the_target_analogy


I wondered if it would be a useful addition to also discuss the differences between 'exact' and 'precise'. I know the two words are near synonyms in colloquial use (as discussed in a thread recently) but is there a difference in meaning in the field of mathematics (and so engineering)?