Special Topics - Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics - page 231

3. Understandable
a.

Percent of Pages with
Accessibility Errors

Make text readable and understandable.

b. Make content appear and operate in predictable

ways.

0 Errors
10%

c. Help users avoid and correct mistakes.

>10 Errors
25%

4. Robust
a.

1-2 Errors
12%

Maximize compatibility with current and future
user tools.

One way of quantifying how well a website meets these criteria
is to assess how many of the pages in the site fail one or more of
each of these guidelines.

3-10 Errors
53%

Some automated tools can check for certain obvious vio-
lations of these guidelines (e.g., missing “Alt” text on image).
Although the errors they detect are generally true errors, they also
commonly miss many errors. Many of the items that the auto-
mated tools flag as warnings may in fact be true errors, but it takes
a human to find out. For example, if an image on a web page has
null Alt text defined (ALT=“”), that may be an error if the image is informational
or it may be correct if the image is purely decorative. The bottom line is that the
only really accurate way to determine whether accessibility guidelines have been
met is by manual inspection of the code or by evaluation using a screen reader
or other appropriate assistive technology. Often both techniques are needed.

Figure 9.10 Results of analysis of a
website against the WCAG guidelines.

AUTOMATED ACCESSIBILITY-CHECKING TOOLS

Some of the tools available for checking web pages for accessibility errors include the
following:

• CynthiaSays( http://www.contentquality.com/ )

• AccessibilityValetDemonstrator( http://valet.webthing.com/access/url.html )

• WebAIM'sWAVEtool( http://wave.webaim.org/ )

• UniversityofTorontoWebAccessibilityChecker( http://achecker.ca/checker/ )

• TAWWebAccessibilityTest( http://www.webdevstuff.com/103/taw-web-

accessibility-test.html )

Once you've analyzed the pages against the accessibility criteria, one way of
summarizing the results is to count the number of pages with errors. For example,
Figure 9.10 shows results of a hypothetical analysis of a website against the WCAG
guidelines. This shows that only 10% of the pages have no errors, whereas 25%
have more than 10 errors. The majority (53%) have 3-10 errors.

In the United States, another important set of accessibility guidelines is the
so-called Section 508 guidelines or, technically, the 1998 Amendment to Section
508 of the 1973 Rehabilitation Act (Section 508, 1998; also see Mueller, 2003).