def skew(m=nil)
      m ||= self.mean
      th = self.reduce(0){|memo, val| memo + ((val - m)**3)}
      th/((self.length)*self.sd(m)**3)
    end
def skew(m=nil)
      m ||= self.mean
      th = self.reduce(0){|memo, val| memo + ((val - m)**3)}
      th/((self.length)*self.sd(m)**3)
    end
report_building(b) would not be implemented
 sample_with_replacement
 sample_without_replacement
 set_valid_data