public static function average( $arr, $key = null ) 
	{
		if( !is_array( $arr ) ) 
		{
			throw new \InvalidArgumentException('CCArr::average - first argunent has to be an array.');
		}

		if ( is_string( $key ) && CCArr::is_multi( $arr ) ) 
		{
			$arr = CCArr::pick( $key, $arr );
		}

		return ( static::sum( $arr ) / count( $arr ) );
	}