  Recent analyses of cosmic ray arrival directions have resulted in evidence
for a positive correlation with active galactic nuclei positions that has weak
significance against an isotropic source distribution. In this paper, we
explore the sample size needed to measure a highly statistically significant
correlation to a parent source catalogue. We compare several scenarios for the
directional scattering of ultra-high energy cosmic rays given our current
knowledge of the galactic and intergalactic magnetic fields. We find
significant correlations are possible for a sample of $>$1000 cosmic ray
protons with energies above 60 EeV.
