#
# Written by:
# -- 
# John L. Weatherwax                2009-04-21
# 
# email: wax@alum.mit.edu
# 
# Please send comments and especially bug reports to the
# above email address.
# 
#-----

library(DAAG)

sapply( cuckoos, is.factor ) 

specname = levels( cuckoos$species )

ss = 0
ndf = 0
for( nam in specname ){
  lgth = cuckoos$length[ cuckoos$species==nam ]
  ss = ss + sum( ( lgth - mean(lgth) )^2 )
  ndf = ndf + length(lgth)-1
}
sqrt( ss / ndf )


diffs = unlist( sapply( split( cuckoos$length, cuckoos$species ), function(x) x - mean(x) ) )
df = unlist( sapply( split( cuckoos$length, cuckoos$species ), function(x) length(x)-1 ) )

sqrt( sum( diffs^2 ) / sum(df) )