validate.covar <- function(null, resp.var){
# give a warning is some levels of factor have small sample size
check.small.level(null, resp.var)
# remove dummy variables with SD = 0
# Note that we need to excluded samples with missing at covariates
# so some covariates having different values may become constant after removing some samples
# we therefore update data frame 'null' by removing those columns
null <- remove.const.covar(null, resp.var)
# check multicollinearity
null <- remove.redendant.covar(null, resp.var)
null
}
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