#
# 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.
#
#-----

fourth_spread = function(X){
    # 
    # Computes the fourth spread.
    #
    # Note that the median is included in both halfs if n is odd.
    #
    tilde_mu = median(X)
    lower_fourth = median( X[ X<=tilde_mu ] )
    upper_fourth = median( X[ X>=tilde_mu ] )
    fs = upper_fourth - lower_fourth
    fs
}