The statistical properties of galaxy morphological types in compact   groups of Main galaxies from the SDSS Data Release 4
  In order to explore the statistical properties of galaxy morphological types in compact groups (CGs), we construct a random group sample which has the same distributions of redshift and number of member galaxies as those of the CG sample. It turns out that the proportion of early-type galaxies in different redshift bins for the CG sample is statistically higher than that for random group sample, and with growing redshift z this kind of difference becomes more significant. This may be due to the existence of interactions and mergers within a significant fraction of SDSS CGs. We also compare statistical results of CGs with those of more compact groups and pairs, but do not observe as large statistical difference as Hickson (1982)'results. 
Xin-Fa Deng, Peng Jiang, Cong-Gen He, Cheng-Hong Luo, Ji-Zhou He