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## gammaStat.R (2009-05-10)
## Gamma-Statistic of Pybus and Harvey
## Copyright 2002-2009 Emmanuel Paradis
## This file is part of the R-package `ape'.
## See the file ../COPYING for licensing issues.
gammaStat <- function(phy)
{
if (!inherits(phy, "phylo")) stop('object "phy" is not of class "phylo"')
N <- length(phy$tip.label)
bt <- sort(branching.times(phy))
g <- rev(c(bt[1], diff(bt))) # internode intervals are from past to present
ST <- sum((2:N) * g)
stat <- sum(cumsum((2:(N - 1)) * g[-(N - 1)]))/(N - 2)
m <- ST/2
s <- ST * sqrt(1/(12 * (N - 2)))
(stat - m)/s
}
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