code: public function f()
      {
          if(is_null($this->arr_f)){
              $arr = $this->frequency();
              array_walk(
                  $arr, 
                  function(&$v, $k, $n){
                      $v = $v / $n;
                  }, 
                  count($this)
              );
  
              $this->arr_f = $arr;
          }
  
          return $this->arr_f;
      }
docs: Relative frequency, like frequency, but divide each but total amount.
  
  @access public
  @return array