Essentials of Ecology

darts are to each other. Note that good precision

is necessary for accuracy but does not guarantee

it. Three closely spaced darts may be far from

the bull’s-eye.

A dartboard analogy (Figure 1) shows the

difference between precision and accuracy. Ac-

curacy depends on how close the darts are to the

bull’s-eye. Precision depends on how close the

SUPPLEMENT 1 S3

Uncertainty, Accuracy,
and Precision in Scientifi c
Measurements

How do we know whether a scientifi c measure-
ment is correct? All scientifi c observations and
measurements have some degree of uncertainty
because people and measuring devices are not
perfect.
However, scientists take great pains to reduce
the errors in observations and measurements by
using standard procedures and testing (calibrat-
ing) measuring devices. They also repeat their
measurements several times, and then fi nd the
average value of these measurements.
It is important to distinguish between
accuracy and precision when determining
the uncertainty involved in a measurement.
Accuracy is how well a measurement conforms
to the accepted correct value for the measured
quantity, based on many careful measurements
made over a long time. Precision is a measure
of reproducibility, or how closely a series of
measurements of the same quantity agree with
one another.

Good accuracy

and good precision

Poor accuracy

and poor precision

Poor accuracy

and good precision

Figure 1 The distinction between accuracy and precision. In scientific measurements, a measuring de-

vice that has not been calibrated to determine its accuracy may give precise or reproducible results that

are not accurate.