  In the past few years, several independent collaborations have presented
cosmological constraints from tomographic cosmic shear analyses. These analyses
differ in many aspects: the datasets, the shear and photometric redshift
estimation algorithms, the theory model assumptions, and the inference
pipelines. To assess the robustness of the existing cosmic shear results, we
present in this paper a unified analysis of four of the recent cosmic shear
surveys: the Deep Lens Survey (DLS), the Canada-France-Hawaii Telescope Lensing
Survey (CFHTLenS), the Science Verification data from the Dark Energy Survey
(DES-SV), and the 450 deg$^{2}$ release of the Kilo-Degree Survey (KiDS-450).
By using a unified pipeline, we show how the cosmological constraints are
sensitive to the various details of the pipeline. We identify several analysis
choices that can shift the cosmological constraints by a significant fraction
of the uncertainties. For our fiducial analysis choice, considering a Gaussian
covariance, conservative scale cuts, assuming no baryonic feedback
contamination, identical cosmological parameter priors and intrinsic alignment
treatments, we find the constraints (mean, 16% and 84% confidence intervals) on
the parameter $S_{8}\equiv \sigma_{8}(\Omega_{\rm m}/0.3)^{0.5}$ to be
$S_{8}=0.94_{-0.045}^{+0.046}$ (DLS), $0.66_{-0.071}^{+0.070}$ (CFHTLenS),
$0.84_{-0.061}^{+0.062}$ (DES-SV) and $0.76_{-0.049}^{+0.048}$ (KiDS-450). From
the goodness-of-fit and the Bayesian evidence ratio, we determine that amongst
the four surveys, the two more recent surveys, DES-SV and KiDS-450, have
acceptable goodness-of-fit and are consistent with each other. The combined
constraints are $S_{8}=0.79^{+0.042}_{-0.041}$, which is in good agreement with
the first year of DES cosmic shear results and recent CMB constraints from the
Planck satellite.
