  Measurements of a well-characterised standard sample can verify the
performance of an instrument. Typically, small-angle neutron scattering
instruments are used to investigate a wide range of samples and may often be
used in a number of configurations. Appropriate standard samples are useful to
test different aspects of the performance of hardware as well as that of the
data reduction and analysis software. Measurements on a number of instruments
with different intrinsic characteristics and designs in a round robin can not
only better characterise the performance for a wider range of conditions but
also, perhaps more importantly, reveal the limits of the current state of the
art of small-angle scattering. The exercise, followed by detailed analysis,
tests the limits of current understanding as well as uncovers often forgotten
assumptions, simplifications and approximations that underpin the current
practice of the technique. This paper describes measurements of polystyrene
latex, radius 72 nm with a number of instruments. Scattering from monodisperse,
uniform spherical particles is simple to calculate and displays sharp minima.
Such data test the calibrations of intensity, wavelength and resolution as well
as the detector response. Smoothing due to resolution, multiple scattering and
polydispersity has been determined. Sources of uncertainty are often related to
systematic deviations and calibrations rather than random counting errors. The
study has prompted development of software to treat modest multiple scattering
and to better model the instrument resolution. These measurements also allow
checks of data reduction algorithms and have identified how they can be
improved. The reproducibility and the reliability of instruments and the
accuracy of parameters derived from the data are described.
