For a complex data table, use mark-up to associate data cells and header cells.

(see guidelines for overcoming access barriers)

Complex data tables are those that have two or more logical levels of row or column headers.

There are several options available to handle complex data tables. The HTML tags you use will depend on the type of table you are creating. Here is a summary of the tags used and links to good examples of how to use the tags. Examples and more information can be found on the W3C site http://www.w3.org/TR/REC-html40/struct/tables.html - h-11.4.

Headers and id. Associate your headers with data by naming your <TH> with the id= attribute then referring to that id using <TD headers="id name">.
thead, tfoot, tbody. Use these elements to group together rows or columns and give them a special designation. The table head and table foot contain information about the table's columns. The table body should contain rows of table data.

For more examples of how to handle complex tables:

http://www.w3.org/TR/REC-html40/struct/tables.html#h-11.4

http://www.webaim.org/techniques/tables/

http://www.macromedia.com/resources/accessibility/dw8/tables.html

Example:

Using mark-up to associate data cells and header cells:

Plants		Animals
Conifers	Flowering	Reptiles	Mammals
Pine	Eucalypt	Lizard	Cat
Cypress	Azalea	Crocodile	Dog

Code for using mark-up to associate data cells and header cells


<table summary="The Plants category is divided into Conifers and Flowering 
and the Animals category is divided into reptiles and mammals">
  <thead>
	<tr>
		<th id="c1-c2" colspan="2"><h2>Plants</h2></th>
		<th id="c3-c4" colspan="2"><h2>Animals</h2></th>
	</tr>
	<tr>
		<th id="c1"><h3>Conifers</h3></th>
		<th id="c2"><h3>Flowering</h3></th>
		<th id="c3"><h3>Reptiles</h3></th>
		<th id="c4"><h3>Mammals</h3></th>
	</tr>
 </thead>
 <tbody>
	<tr>
		<td headers="c1-c2 c1">Pine</td>
		<td headers="c1-c2 c2">Eucalypt</td>
		<td headers="c3-c4 c3">Lizard</td>
		<td headers="c3-c4 c4">Cat</td>
	</tr>
	<tr>
		<td headers="c1-c2 c1">Cypress</td>
		<td headers="c1-c2 c2">Azalea</td>
		<td headers="c3-c4 c3">Crocodile</td>
		<td headers="c3-c4 c4">Dog</td>
	</tr>
 </tbody>
</table>