  We investigate claims of an anomalously large amplitude of the dipole in the
distribution of quasars on the sky. Two main issues indicate that the
systematic uncertainties in the derived quasar-density dipole are
underestimated. Firstly, the spatial distribution of the quasars is not a pure
dipole, possessing low-order multipoles of comparable size to the dipole. These
multipoles are unexpected and presumably caused by unknown systematic effects;
we cannot be confident that the dipole amplitude is not also affected by the
same systematics until the origin of these fluctuations is understood.
Secondly, the 50 percent sky cut associated with the quasar catalogue strongly
couples the multipoles, meaning that the power estimate at ell=1 contains
significant contributions from ell>1. In particular, the dominant quadrupole
mode in the Galactic mask strongly couples the dipole with the octupole,
leading to a large uncertainty in the dipole amplitude. Together these issues
mean that the dipole in the quasar catalogue has an uncertainty large enough
that consistency with the cosmic microwave background (CMB) dipole cannot be
ruled out. More generally, current data sets are insufficiently clean to
robustly measure the quasar dipole and future studies will require samples that
are larger (preferably covering more of the sky) and free of systematic effects
to make strong claims regarding their consistency with the CMB dipole.
