// add parser through the tablesorter addParser method
	$.tablesorter.addParser({
		// set a unique id
		id: 'grades',
		is: function(s) {
			// return false so this parser is not auto detected
			return false;
		},
		format: function(s) {
			// format your data for normalization
			return s.toLowerCase().replace(/good/,2).replace(/medium/,1).replace(/bad/,0);
		},
		// set type, either numeric or text
		type: 'numeric'
	});
	
	$(function() {
		$("table").tablesorter({
			headers: {
				6: {
					sorter:'grades'
				}
			}
		});
	}); 				
	

<table cellspacing="1" class="tablesorter">			
	<thead>>
		<tr>
			<th>name</th>
			<th>major</th>
			<th>gender</th>
			<th>english</th>
			<th>japanese</th>
			<th>calculus</th>
			<th>overall grades</th>
		</tr>
	</thead>
	<tbody>
		<tr>
			<td>student01</td>
			<td>languages</td>
			<td>male</td>
			<td>80</td>
			<td>70</td>
			<td>75</td>
			<td>bad</td>
		</tr>
		<tr>
			<td>student02</td>
			<td>mathematics</td>
			<td>male</td>
			<td>90</td>
			<td>88</td>
			<td>100</td>
			<td>good</td>
		</tr>
		<tr>
			<td>student03</td>
			<td>languages</td>
			<td>female</td>
			<td>85</td>
			<td>95</td>
			<td>80</td>
			<td>medium</td>
		</tr>
	</tbody>
</table>