  Simple analytic expressions are derived for the sensitivity curve of a pulsar
timing array (PTA) to both a monochromatic source of gravitational waves and an
isotropic stochastic background of gravitational waves. These derivations are
performed in both frequentist and Bayesian frameworks and the results verified
with numerical injections and recovery in mock PTA datasets. The consistency of
the frequentist and Bayesian approaches is demonstrated and the results are
used to emphasise the fact that the sensitivity curve of a PTA depends not only
on the properties of the PTA itself, but also on the properties of the
gravitational wave source being observed.
