Re: Your comments on WCAG 2.0 Last Call Working Draft of December, 2007

On Mon, Mar 31, 2008 at 11:39 AM, Christophe Strobbe (on behalf of
BenToWeb) <christophe.strobbe@esat.kuleuven.be> wrote:
>
>  [To: public-comments-WCAG20@w3.org; BCC: BenToWeb Consortium]
>
>  After discussion on the BenToWeb mailing list, the consortium is satisfied
> with all the responses except for the following:
>  Comment 3: contrast algorithm for 1.4.3: Contrast (Minimum)
>  Source:
> <http://lists.w3.org/Archives/Public/public-comments-wcag20/2008Feb/0084.html>
>  (Issue ID: 2535)
>
>  The original comment was based on research that is reported in a project
> deliverable at
> <http://webcc.fit.fraunhofer.de/downloads/projects/bentoweb/deliverables/BenToWeb_D3.5_rev.pdf>
> but which was not yet publicly available when the original comment was
> written.
>
>  This research investigated equations for calculating colour contrast and
> appropriate minimum levels of contrast.
>  Four diferent sets of equations were investigated:
>
>  * WAI Accessibility Evaluation and Repair Tools (Ridpath and Chisholm,
> 2000), brightness equations ("AERT/B"),
>
>  * WAI Accessibility Evaluation and Repair Tools (Ridpath and Chisholm,
> 2000), colour contrast equations ("AERT/CC"),
>
>  * W. S. Thune (2003) [3] has provided a series of equations, known as the
> Catman equations, using linear, quadratic and cubic functions (Catman1,
> Catman2, Catman3),
>
>  * WCAG 2.0 ­ Working Draft 17 May 2007: contrast ratio definition (WCAG2).
>
>  No evidence-based support for these ratios and equations could be found in
> the literature. (The references in Understanding SC 1.4.3 [4]) mostly
> predate WCAG 1.0; it is not clear from the text whether any of these
> references is the source of the proposed colour contrast equation or not.)
>
>  As providing user evidence from across the entire colour spectrum could
> require a very large amount of user testing, it was decided to use a sample
> of colour combinations that are commonly used on the Web for text and
> background combinations. Therefore a survey of 100 popular and important
> websites was undertaken to provide information about common colour
> combinations and the 12 most popular combinations were used in the main
> testing. This set of combinations was supplemented with a number of
> subcombinations, which resulted in a total of 54 combinations (see 3.4.4 -
> Colour combinations used in stimuli). These were presented in a random
> sequence.
>
>  In the user testing, 165 people with full colour vision each viewed 54
> simple web pages showing a single sentence in one colour on a different
> coloured background (more details on the methodology can be found in chapter
> 3 of the report). Effects of font type and size were also investigated by
> presenting the sentences in either Arial (a sans serif font) or Times New
> Roman (a serif font) and in either 10, 12 or 14 point. Participants were
> asked to rate how easy the sentence was to read on a 1-5 Lickert scale and
> to comment on their rating.
>
>  Font type and size, as varied in this study, had little impact on the
> results, accounting for only 4% of the variance in user ratings.
>
>  Colour contrast accounted for approximately 20-40% of the variance (a
> significant and substantial proportion), depending on the equations used. (A
> series of non-linear regressions was conducted, using the different
> equations to predict the ease of reading ratings. In each case, linear,
> quadratic, and cubic equations were used in the prediction, to investigate
> which form of relationship accounted for the greatest proportion of the
> variance. Table 4.1 in the report summarizes the results of the
> regressions.) The equation which accounted for the greatest proportion of
> the variance in user ratings was the AERT/B equation when used in a cubic
> relationship to user ratings.
>
>  Using this equation, thresholds for adequate contrast between text and
> background were calculated, and the appropriateness of the 54 colour
> combinations used in the experiment calculated to provide initial examples
> of good and bad contrast.
>  Section 4.4.4 - Readable threshold values - contains thresholds for each of
> the equations. The "readable threshold value" for AERT/B is 66.8; the
> "highly readable threshold value" is 85.5.
>
>
>  [1] <http://www.w3.org/TR/WCAG20/#contrast-ratiodef>
>
>  [2] <http://www.w3.org/TR/AERT#color-contrast>
>
>  [3] Originally availabe at
> <http://www.iastate.edu/~class.12003.engl.313/cps/contrast.html> (URL is now
> dead); now at
> <http://www.public.iastate.edu/~class.12003.engl.313/cps/contrast.html>.
>
>  [4]
> <http://www.w3.org/TR/2007/WD-UNDERSTANDING-WCAG20-20071211/complete.html#visual-audio-contrast-contrast>
>
>  Best regards,
>
>  Christophe Strobbe
>  on behalf of BenToWeb
>
>
>  PS: Apparently, ANSI/HFS 100-1988, American National Standard for Human
> Factors Engineering of Visual Display Terminal Workstations  has been
> replaced by "ANSI/HFES 100-2007, Human Factors Engineering of Computer
> Workstations"
> <http://www.hfes.org/Publications/ProductDetail.aspx?ProductId=69>.
>

---------------------------------------------
Response from Working Group:
---------------------------------------------

Thank you for correcting the formula in the report. But we note that
all of the subjects in the study had normal color vision. WCAG 2.0 is
not focused on selecting the best contrast for people without
disabilities, but rather for those with disabilities. The WCAG 2.0
formula was chosen to maximize relative luminance contrast so that the
contrast would be effective with various color perception differences.

We expect that there will be user tests and a fuller paper describing
derivation of these formulas. These resources will be added to the
Understanding document as they become available.

Thanks again for the interest that you have taken in these guidelines.
Could we ask you to let us know whether or not you are satisfied with
this response by Wed, April 16?

Regards,

Loretta Guarino Reid, WCAG WG Co-Chair
Gregg Vanderheiden, WCAG WG Co-Chair
Michael Cooper, WCAG WG Staff Contact

On behalf of the WCAG Working Group

Received on Friday, 11 April 2008 17:23:40 UTC