public function setMinViewability($value = null)
    {
        switch ($value) {
            case 'full_view':
                $this->_params['min-viewability'] = 'full';
                break;
            case 'partial_view':
                $this->_params['min-viewability'] = 'partial';
                break;
            case null:
                unset($this->_params['min-viewability']);
                break;
        }
        return $this;
    }