A matrix that shows the relationship between two classes of objects. Horner-Devine et al. (2007) Ecology 88, 6, 1345, who provide crucial help for the mathematically challenged, use presence–absence matrix in which each row represents a species or taxon, and each column represents a sampling site. Within each incidence matrix the presence of a species, taxon, or operational taxonomic unit (OTU) at a specific site is denoted by a 1, and its absence is denoted by a 0. Williamson (1978) J. Ecol. 66, 911 is recommended.