There isn't one. As a proxy, one could do a bunch of data gymnastics based off of the number of times a machine+account uses a single IP address and display resolution + OS. It's a pretty bad approximation though.
Is there a way to detect webcam model? That's the one unique thing I can think of.. A lot of desktops don't have them and if they do its like logitech whatever model.
They're dividing by race, income and level of education. Did they get all that from their models too? I assumed it was a survey.