  Probability density functions (PDF) of statistical distributions of cluster
sizes N, where N is the number of particles in the cluster, often seem to have
less freedom than expected from considering the number of degrees of freedom at
the clusters' source. The full width at half maximum appears to be comparable
to the average <N>. Such a hidden symmetry is intriguing theoretically and
practically impairs size selection towards narrow distributions. However,
reviewing the example of Helium cluster beams demonstrates that the origin of
the apparent fixing is the assumption that the distributions should be
log-normal or exponential and the subsequent use of these functions to fit the
data in n = ln(N) log-space. This demands more care when using parametric
statistics. Alternatives to the traditionally employed fitting functions are
discussed.
