Is it possible (and How?) to track people with disabilities on websites? I want to know how much people on my website use a screen reader, use plugins for easier reading and surfing on websites, use a text browser or use else native configurations/programs for easier using the web.
Short answer - it is not possible, and even if it was, it is definitely not recommended:
I can give you many more links that explain why this is not recommended if you are genuinely interested in this, but the bottom line is even the WC3 WCAG2.0 guidelines do not recommend doing this.
My previous answer was intended to answer the why question, but obviously you asked a how question. I will gently remind you that this is a User Experience Stack Overflow and how questions are typically regarded as off topic.
However, Accessibility related questions tend to bend the rule a little, so...
The most common screen readers are: http://webaim.org/projects/screenreadersurvey5/#used
However, accessibility is a huge subject and there is no way to detect the OS accessibility features like High Contrast in Windows, no way to detect alternate pointer devices, no way to discern whether a keyboard user is an accessibility user using JAWS from a normal keyboard user.
So in summary, there really isn't much you can detect, and there is little value in spending time and money on trying to do this.
This article discusses the merits of part of this subject - detecting screen reader usage. It does not cover detection of other assistive technology like adaptive keyboards and alternate point devices, or replacing your CSS style sheets, or activating the high contrast OS visual scheme, etc.
Breaking down the Question
Looking at this question with screen readers in mind, it breaks down into two questions:
- What are the advantages to detecting screen readers?
- What are the disadvantages to detecting screen readers?
Obviously there’s the potential to provide a better experience to users with screen readers because you know they’re there. And there’s the ability to gather statistics on screen reader usage in a way we’ve never been able to before.
The risk that screen reader users will just get shrugged off into a simpler, poorly-updated version of the site because nobody can be bothered to create and maintain an equal experience. And there’s the ability to gather statistics on screen reader usage in a way we’ve never been able to before.
Funny how the second sentence is the same in both cases...
Although screen reader detection may be possible, though discouraged, it's only a little subset of all the possible "user with disabilities" scenarios.
A huge proportion of the people that benefit when we try to make sites accessible are the elderly. They may just take a little longer to do things, struggle to read tiny text and have less fine motor control to click on little targets. You'd never ever pick that up with detection.
If you were to try detection, you'd end up with a much lower percentage of users being flagged than the reality.
When you re-frame accessibility from disabilities to just people who struggle with the web, you have a load more use cases like people with their arm in a sling and people who forgot to bring their mouse to a meeting!