open(FILE, $data_file) or die("cannot open file :  $!");
	print "<p>opened file $data_file ok\n";
	$a = 0;
	while ($line = <FILE>) {
		$a++;
		#record every thousand lines as way to check progress
		if ($a % 1000 == 0) {
			open(pFILE, '>>progress.txt') or die("cannot open progress file : $!");
			print pFILE ", $a";
			close(pFILE);
		}
		#parse the line
		@line = split(//, $line);
		my $line_id = join('', $line[6], $line[7], $line[8]);

		if ($line_id  eq $target_line_id) {
			my $respondent = join('', $line[0], $line[1], $line[2], $line[3]);
			# if this line meets the criteria
			if (($line[$col_1 - 1] eq $value_1) && ($line[$col_2 - 1] eq $value_2)) {
				$file = ">>".$data_dir."/t".$rid;
				#record this respondent in a text file
				open(rFILE, $file) or dienice("cannot open file : $_[0] $!");
				print rFILE "$respondent\n";
				close(rFILE);
				# output progress to browser
				print ", $respondent";
			}
		}
	}
	close(FILE);