  We investigate the importance of projection effects in the identification of
galaxy clusters in 2D galaxy maps and their effect on the estimation of cluster
velocity dispersions. A volume limited galaxy catalogue that was derived from a
Standard CDM N-body simulation was used. We select clusters using criteria that
match those employed in the construction of real cluster catalogues and find
that our mock Abell cluster catalogues are heavily contaminated and incomplete.
Over one third (34 per cent) of clusters of richness class R>=1 are
miclassifications arising from the projection of one or more clumps onto an
intrinsically poor cluster. Conversely, 32 per cent of intrinsically rich
clusters are missed from the R>=1 catalogues, mostly because of statistical
fluctuations in the background count. Selection by X-ray luminosity rather than
optical richness reduces, but does not completely eliminate, these problems.
Contamination by unvirialised sub-clumps near a cluster leads to a considerable
overestimation of the cluster velocity dispersion, even if the analysis is
restricted only to galaxies close to the cluster centre. Thus, the distribution
of cluster masses - often used to test cosmological models - is a highly
unreliable statistic. However, the median value of the distribution is
considerably more robust. Improved estimates of the cluster velocity dispersion
distribution require constructing new cluster catalogues in which clusters are
defined according to the number of galaxies within a radius about three times
smaller than the Abell radius.
