$start_1 = new Benchmark;
	for $i (0 .. $iterations) {
		$table = "<".$db_dir."/r".$response_id;
	      	open(FILE, $table) or dienice("cannot open file : $table $!");
		while ($data = <FILE>) {
			$union{$data} = undef;
		}
		close(FILE);
		$table = "<".$db_dir."/r6000";
	      	open(FILE, $table) or dienice("cannot open file : $table $!");
		while ($data = <FILE>) {
			$isect{$data} = undef if exists $union{$data};
		}
		close(FILE);
		@isect = keys %isect;
		$response_id++;
	}
	$end_1 = new Benchmark;
	$diff = timediff($end_1, $start_1);