validation : elements selected for validation (complementary to the
calibration set)
Arguments
obs
a vector containing binary values (either 0 or 1)
ratio
a numeric between 0 and 1 corresponding to the proportion
of obs values to sample
as.logical
(optional, defaultFALSE)
A logical value defining whether output should be returned as a vector of
TRUE/FALSE values or integer values corresponding to the indices of obs
elements to be kept
seedval
(optional, defaultNULL)
An integer value corresponding to the new seed value to be set