def dichotomize(low=nil)
          low ||= factors.min

          recode do |x|
            if x.nil?
              nil
            elsif x > low
              1
            else
              0
            end
          end
        end
Dichotomize the vector with 0 and 1, based on lowest value.
If parameter is defined, this value and lower will be 0
and higher, 1.