  Hard-sphere colloids are popular as models for testing fundamental theories
in condensed matter and statistical physics, from crystal nucleation to the
glass transition. A single parameter, the volume fraction (phi), characterizes
an ideal, monodisperse hard-sphere suspension. In comparing experiments with
theories and simulation, researchers to date have paid little attention to
likely uncertainties in experimentally-quoted phi values. We critically review
the experimental measurement of phi in hard-sphere colloids, and show that
while statistical uncertainties in comparing relative values of phi can be as
low as 0.0001, systematic errors of 3-6% are probably unavoidable. The
consequences of this are illustrated by way of a case study comparing
literature data sets on hard-sphere viscosity and diffusion.
