  The points at which the log likelihood falls by 1/2 from its maximum value
are often used to give the `errors' on a result, i.e. the 68% central
confidence interval. The validity of this is examined for two simple cases: a
lifetime measurement and a Poisson measurement. Results are compared with the
exact Neyman construction and with the simple Bartlett approximation. It is
shown that the accuracy of the log likelihood method is poor, and the Bartlett
construction explains why it is flawed.
