  The Shear TEsting Programme, STEP, is a collaborative project to improve the
accuracy and reliability of all weak lensing measurements in preparation for
the next generation of wide-field surveys. In this first STEP paper we present
the results of a blind analysis of simulated ground-based observations of
relatively simple galaxy morphologies. The most successful methods are shown to
achieve percent level accuracy. From the cosmic shear pipelines that have been
used to constrain cosmology, we find weak lensing shear measured to an accuracy
that is within the statistical errors of current weak lensing analyses, with
shear measurements accurate to better than 7%. The dominant source of
measurement error is shown to arise from calibration uncertainties where the
measured shear is over or under-estimated by a constant multiplicative factor.
This is of concern as calibration errors cannot be detected through standard
diagnostic tests. The measured calibration errors appear to result from stellar
contamination, false object detection, the shear measurement method itself,
selection bias and/or the use of biased weights. Additive systematics (false
detections of shear) resulting from residual point-spread function anisotropy
are, in most cases, reduced to below an equivalent shear of 0.001, an order of
magnitude below cosmic shear distortions on the scales probed by current
surveys.
  Our results provide a snapshot view of the accuracy of current ground-based
weak lensing methods and a benchmark upon which we can improve. To this end we
provide descriptions of each method tested and include details of the eight
different implementations of the commonly used Kaiser, Squires and Broadhurst
(1995) method (KSB+) to aid the improvement of future KSB+ analyses.
