It had estimates of the wage/income of site visitors.
Sorry - you were asking about Quantcast while I was thinking about Compete’s data.
From personal perspective (I have a very large customer db along with extensive customer surveys, so I know my site visitors extremely well) Quantcast’s numbers appear very accurate on the demographic front. Actually the demographics appear dead-on in many respects while the visitor volume is the metric that is more fluky - you’d expect it to be the other way around, but in my case it is not.
I use Quantcast when reviewing sites for media placements and “reality checks” - qualitatively it appears quite good, while it the brand name is geared toward quantitative analysis. Funny.