Appearance event ordination or AEO is a scientific method for biochronology through the ordering of the appearance of fossil mammal genera by multivariate analysis, using conjunctional (overlapping) and disconjunctional (nonoverlapping) range distributions in large sets of data.
[1] AEO is based on faunal overlap and stratigraphic superposition to derive a best-fit sequence of first and last appearance events.
A multivariate ordination algorithm is applied to derive a first-pass, hypothesized sequence of first and last appearances.
The criterion basically seeks to pull apart as many hypothesized age range overlaps as possible, especially if they involve common taxa.
[4] Appearance event ordination uses objective, explicit, recordable, repeatable, and quantitative analyses of faunal and biostratigraphic data to arrive at a conclusion, according to John Alroy.