Guideline 4
Provide access to data in tables for blind users.
Reading and manipulating tables is an important way of processing scientific information and is a particular problem for blind users. Using data in a table requires referring to the headings for each row and column in order to interpret the information in a single cell. When navigating tables, blind users often don't even know what cell they are in at any time, or the column and row headers. A standard method for providing programmatic information about reading location and which headers apply to each cell will provide compatibility with assistive technologies that support the standard. One such standard is included as part of the W3C's HTML 4.0 (or higher) specification.
Checkpoint 4.1
Design all HTML-based tables in accordance with the Web Content Accessibility Guidelines published by the World Wide Web Consortium's Web Accessibility Initiative (W3C/WAI).
Producing data tables in conformance with the Web Content Accessibility Guidelines exposes crucial information about the structure of a table.
Technique 4.1.1
Use HTML to mark up tables.
Here is a simple HTML table, taken from the PIVoT Web site.
Below is part of the HTML for this table. Note the use of the tags <th>, used to identify each table header, and the attribute <scope>, used to differentiate table headers in each row and column. Also note that <summary> is used to provide a brief description of the table itself.
<table summary="HTML representation of Table 2.3: Variation of g with Latitude." border="1"><caption>Table 2.3: Variation of g with Latitude</caption><tr><th scope="col">Station</th><th scope="col">Latitude</th><th scope="col">g</th></tr><tr><th scope="row" align="left">Quito, Ecuador</th><td>zero degrees N</td><td>nine point seven eight zero m slash s sup two base</td></tr><tr><th scope="row" align="left">Madras, India</th><td>one three degrees N</td><td>nine point seven eight three m slash s sup two base</td></tr><tr><th scope="row" align="left">Hong Kong</th><td>two two degrees N</td><td>nine point seven eight eight m slash s sup two base</td></tr>Someone using a screen reader that is capable of properly reading table markup will first hear the table summary read aloud ("HTML representation of Table 2.3: Variation of g with Latitude"), then the headers ("Station, Latitude, g"), then the information in the data cells. The user can press specific key combinations to navigate each table cell, receiving feedback about position relative to each row and column header.
Table information can also be presented in a text-only format, where the data are presented linearly. This approach requires that the table be summarized and that headers be identified at the beginning of the presentation. The headers must also be associated with each piece of relevant information. Here is a linear presentation of part of the same data table shown above.
<p>HTML data table description of Table 2.3: Variation of g with Latitude</p><br><h3><a name="plaintext">Plain-text description of table 2.3: Variation of g with Latitude</a></h3><p>The table has three headers: Station, Latitude and g</p><p>Station: Quito, Ecuador; Latitude: zero degrees N; g: nine point seven eight zero m slash s sup two base</p><p>Station: Madras, India; Latitude: one three degree N; g: nine point seven eight three m slash s sup two base</p><p>Station: Hong Kong; Latitude: two two degree N; g: nine point seven eight eight m slash s sup two base</p>Here is what this table looks like in a browser:
Technique 4.1.2
Provide alternative access to static tables.
For tabular data that the user cannot change, preproduced audio can provide useful access to tables. The entire data table can be read aloud, or the equivalent script can be provided in text. Techniques for how best to read tables of data are available from the National Braille Association Tape Recording Manual, or from MathSpeak. (See Appendix 3, Guides to Spoken Mathematics.)
Tables can also be created in braille, though there are formatting challenges, especially for long lines of text. Professional brailling firms can advise the best way to braille tables. (See Appendix 1, Braille and Tactile Graphics Production Resources.) Once created, the set of brailled tables can be made available through the publisher's distribution mechanism, through an arrangement made with the braille production facility or via the course itself. Availability of braille tables should be clearly documented on the Web site, including contact information. Availability of braille tables should also be listed with the American Printing House for the Blind, a central source of information on how educators can locate accessible materials, listed in Appendix 1.