Most previous studies of groups have selected their samples from either optical or X-ray catalogues. Optically selected group catalogues inevitably include a fraction of false groups; chance associations of galaxies, and uncollapsed groups which are still in the process of formation. X-ray selection, based on the detection of the hot intra-group medium, ensures that the groups are fully collapsed, but most X-ray catalogues are based on shallow observations which are likely to miss fainter systems. Unfortunately there is currently no statistically complete sample of X-ray selected groups. The best available samples of galaxy groups are non-statistical collections of systems, chosen because they have X-ray data available, rather than as a representative subset of the group population. This leaves both optical and X-ray selected sample open to unknown biases; addressing the problem requires a well-selected optical sample of groups with high-quality X-ray data.

One possible example of such bias is the prevalence of cool-core systems among the group population. In statistically selected samples of galaxy clusters, roughly 50% of systems have a cool core (significant cooling of the dense gas in the cluster core, e.g., Sanderson et al. 2006). In non-statistical samples of groups, up to 85% of systems are found to have cool cores (Dong et al. 2010). Without a statistically selected sample of groups with X-ray observations, we cannot know whether this indicates a dramatically different balance between heating and cooling, or a strong bias toward detection of groups with the strong central X-ray intensity peak associated with cooling flows (as found in clusters, Eckert et al. 2011). Such a bias could suggest that a large number of X-ray bright, non-cool-core groups have yet to be discovered.