Glossary of "Cladistics (2nd ed.)" by Kitching et al. (1998)
- A taxon used in a cladistic analysis for comparative purposes, usually with respect to character polarity determination. Cf. ingroup.
Glossary of Plant Systematics (1st ed.) by Simpson (2006)
- A taxon or group of taxa that is not a member of the ingroup.
Glossary of "Life (9th ed.)" by Sadava et al. (2011)
- In phylogenetics, a group of organisms used as a point of reference for comparison with the groups of primary interest (the ingroup).
"Phylogenetics" by Wiley (1981), p. 7
- An out-group is a species or higher monophyletic taxon that is examined in the course of a phylogenetic study to determine which of two homologous characters may be inferred to be apomorphic. One or several out-groups may be examined for each decision. The most critical out-group comparisons involve the sister group of the taxon studied.
- Kitching, I. J., Forey, P. L. Humphries, C. J. & Williams, D. M. 1998. Cladistics, 2nd ed.: the Theory and Practice of Parsimony Analysis. -xii + 228 pp. Oxford University Press, New York.
- Simpson, M. G. 2006. Plant Systematics. -xi + 590 pp. Elsevier Academic Press, Oxford.
- Sadava, D., Hillis, D. M., Heller, H. C. & Berenbaum, M. R. 2011. Life: The Science of Biology, 9th ed. -xliv + 1266 pp. Sinauer Associates, Inc., Sunderland.
- Wiley, E. O. 1981. Phylogenetics: the Theory and Practice of Phylogenetic Systematics. -xv + 439 pp. John Wiley & Sons, Inc., New York.