  In order to constraint the limitations of association methods applied to
galaxy surveys, we analysed the catalogue of halos at $z=0$ of a cosmological
simulation, trying to reproduce the limitations that an observational survey
deal with. We focused in the percolation method, usually called Friends of
Friends method, commonly used in literature. The analysis was carried on the
dark matter cosmological simulation MDPL2, from the Multidark project. Results
point to a large fraction of contaminants for massive halos in high density
environments. Thresholds in the association parameters and the subsequent
analysis of observational properties can mitigate the occurrence of fake
positives. The use of tests for substructures can also be efficient in
particular cases.
