epsilon is a measure of sphericity created by Greenhouse and Geiser (1958). The original measure was biased and therefore, Hyunh and Feldt (1976) produced a revised version (note that the 1976 paper contained typos that were uncorrected in SPSS; Lecoutre, 1991)
epsilon(dta, cols)a data.frame
a vector of column names indicating the relevant columns on which to compute epsilon. Any other columns are ignored.
returns the Hyunh-Feldt estimate of sphericity epsilon