What is the Difference between Accuracy, Consistency and Precision?

Question

A discussion has come up with my colleagues between Accuracy, Consistency and Precision.  I realize Accuracy and Precision are synonymous in dictionaries, and I realize this may be semantics for some; but when defining capabilities of an instrument, room for error exists when definitions are misunderstood.  I figured I'd post here to see what people have to say.

Originally, I always heard it as just Accuracy and Consistency, but now Precision has entered the discussion.  I understand that consistency is the ability to do the same thing repeatedly with very little variance, even though it may not be accurate.  I also understand that something may be accurate, but with a large variance that makes it inconsistent.  Where does precision enter the discussion?

Does precision replace consistency, or accuracy; or do all three have their own value?

Thanks

rainwater · Accepted Answer

Accuracy means a measure or value that is GENERALLY SPEAKING correct.

Precision means a measure or value that has a METICULOUSLY DETAILED manner expression of a measure or value. A measure or value is viewed to be precise when it is expressed with an abundance of decimal point or significant value in excess of the desired or target value.

I was baffled with the question of consistency, but yeah, I concur with my predecessor with his definition of consistency. But if you took Analytical/Quantitative Chemistry, you will know that "consistency of measures speaks of a precise measure".

veeyesvee · Answer

Accuracy is the deviation from a true value The true value is as decided by an international standard. It is a wrong statement when you say accuracy is synonymous with resolution. Far from that. Resolution is ability to see a change. 
Let me discuss this with respect to a weighing system. My weight is 70.000kg . However a weigh scale shows that as 69.5kg. that means inaccuracy of weigh scale is 0.5kg. This weigh scale cannot show the difference if I consume a 100mlitre of water, as its resolution itself is 100 gms. So I use another weigh scale that shows my weight as 70.21kg. Its inaccuracy is 0.21kg. If I consume 100gms of water my weight now becomes 70.31kg which is correctly shown by this weigh sacle. What is correct? Not the actual value, but the change it showed is correct. If I now weigh myself when I am in a platform where I also have a diesel generator, my weight keeps changing as observed by the weigh scale. That is inconsistency. In this case, the resolution of the scale (10gm) may not mean much... Thus resolution is taken a 1/3rd of consistency. If the weight itself does not remain constant how can one identify changes as little as 10 gms therein? If change is 100gm, perhaps by averaging a few values you can improve the consistency. baby weighing is a challenge for the same reason, as they are more active whether in weighing plate or outside it!!Movement of the baby changes apparent weight.
 Experienced engineer with over 50 years' experience, 20 in weighing with electronic load sensors.

Skywave · Answer

Accuracy is a term that defines the error that exists in a measurement. 
Precision is the degree of resolution that can be obtained in that measurement. 
Consistency is a measure of the uniformity of repeatability of that measurement.

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What is the Difference between Accuracy, Consistency and Precision?

3 Answers