We ended Example 5.3 with the statement that the method could be continued to obtain
a refined estimate of the root. We must now develop an objective criterion for deciding when
to terminate the method.
An initial suggestion might be to end the calculation when the error falls below some
prespecified level. For instance, in Example 5.3, the true relative error dropped from 12.43
to 0.709% during the course of the computation. We might decide that we should terminate
when the error drops below, say, 0.5%. This strategy is flawed because the error estimates
in the example were based on knowledge of the true root of the function. This would not be
the case in an actual situation because there would be no point in using the method if we
already knew the root.
Therefore, we require an error estimate that is not contingent on foreknowledge of the
root. One way to do this is by estimating an approximate percent relative error as in