In the previous section on
significant
figures we dealt obliquely with the accuracy of an experiment. In this section we'll deal with the
precision of an experiment. In everyday usage the two words have very
similar meanings, but in science there meanings are quite different.
Precision is the degree to which the results of multiple repeat experiments
agree with one another. For instance if an experiment is repeated 3 times and
the same result is obtained all three times, then the result is considered to be
very precise. Accuracy is the degree to which the results of an experiment
agree with the true or known value.
an experiment or set of experiments may be very precise but not accurate. Less
commonly, the results could be accurate but not precise.
An Example
A chemist is asked to determine the concentration of a chemical dissolved in a
solution. The chemist performs the experiment three times for good measure, and
the concentration determined to be 1.74 M, (moles/liter), 1.73 M and 1.75 M.
The average of these numbers is 1.74 M. This result is extremely precise, but
suppose the chemist is not a very good chemist and made the same mistake in all
three experiments: the true concentration of the chemical in solution is 2.32 M.
Even though this experiment was done three times, and the concentration was
determined very precisely, it is not an accurate result.
Now let's say that another chemist performs three more experiments to determine
the concentration of the same chemical in solution, and finds the following
values: 2.87, 1.48 and 2.61 M. When averaged, these values accurately give 2.32
M, but the experiments were not precise. In fact, it may have been lucky that
they averaged out perfectly.
