Unit 4 sec 5.2
Accuracy and precision
13 January 2016
23:39
A word often used in this unit is precision. Elsewhere in
the module. It is used in the context of a number being stated to so many
decimal places or a certain number of significant figures. Its meaning will be
extended here. It is a term that can easily be confused with accuracy, but in fact
these 2 terms have a subtle difference in meaning.
How would you
describe this performance? The answer is that they showed accuracy but not precision.
You can be described as accurate, because the average of the 5 shops is close
to the centre. You are not precise, because the shirts are widely spread.
The 1st
moral of this little tale is that accuracy is a statement about the location of
this set of measurements: the closer the average of the measurements to the
true value of what is being measured, the more accurate your estimation of the
value. The 2nd message is that precision tells you about the spread
of this set of measurements: the more tightly packed your values, the more
precise your measurements are.
Accuracy versus precision
for a set of (repeated) measurements:
¨
accuracy describes how
close the average is to the true value.
¨
Precision describes how
close the measurements are to each other.
Ideally, when making measurements, you would like to have
both accuracy and precision!
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