I stumbled across a question on the User Experience Stack Exchange site today, "Accessibility: How to track peoples 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."

It's not a bad question, after all we track all kinds of things with analytics. Google now has a stab at estimating the gender and interests of your site visitors so this is a logical step on from that.

Now because the questioner doesn't provide us with any more context than the snippet above, I'm going to base my response on the assumption that they want to know in order to influence product decisions.

Just as I check the analytics of a site to see if it still makes sense to support IE7 (hell no), maybe knowing how many users have disabilities could help you with prioritising your development backlog or (unfortunately) deciding whether to bother testing for accessibility at all.

OK, So What Would We Test For?

Now that we think we have captured the mindset of the developer here, let's break this down a bit further, what can we test for in our new, fancy disability detection code?

Things that come to mind:

Screen Reader Use

A pretty obvious one to check for, these users definitely tick the box for needing a site to be accessible.

Navigating With a Keyboard Only

If the user doesn't use the mouse at all, they probably count here too.

Browser Zoomed In

People tend to zoom in if they're struggling to read. Maybe they would use magnification software. Hmm, this is starting to look tricky to test for.

Color Blindness

Oh, um, not sure how we can check this one. It's important though.

Deaf and Hard of Hearing

I guess these people might struggle with some content...

The elderly? Young children? People who have broken their arm? The dad holding his baby while trying to shop?

Once you head off down this train of thought, it's pretty clear why disability detection is a bad idea. 

Accessibility Isn't Solely About Disabilities

It's often described (and I'm totally guilty of this too) as "enabling those with disabilities to use your website".

I like to flip that on it's head and state that it's all about making the web accessible to all. There's a subtle difference in there and it's where people like our dad trying to shop online when holding his baby come in. 

Maybe it's easier for him to just browse using the keyboard and not the mouse. More importantly for your business, maybe he's more likely to buy from your shop if it's easy for him to just navigate with the keyboard.

If you start searching for statistics on who will benefit from you making your site accessible, you're always going to come up with a number way lower than the reality. Loads of people benefit from accessibility. 

You know the right thing to do.