Demo

Name Major Gender English Japanese Calculus Overall grades
Student01 Languages male 80 70 75 bad
Student02 Mathematics male 90 88 100 good
Student03 Languages female 85 95 80 medium

Javascript

	// 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'
				}
			}
		});
	}); 				
	

HTML

<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>