This is for a new feature on http://cssfingerprint.com (see /about for general info).

The feature looks up the sites you've visited in a database of site demographics, and tries to guess what your demographic stats are based on that.

All my demgraphics are in 0..1 probability format, not ratios or absolute numbers or the like.

Essentially, you have a large number of data points that each tend you towards their own demographics. However, just taking the average is poor, because it means that by adding in a lot of generic data, the number goes down.

For example, suppose you've visited sites S0..S50. All except S0 are 48% female; S0 is 100% male. If I'm guessing your gender, I want to have a value close to 100%, not just the 49% that a straight average would give.

Also, consider that most demographics (i.e. everything other than gender) does not have the average at 50%. For example, the average probability of having kids 0-17 is ~37%. The more a given site's demographics are different from this average (e.g. maybe it's a site for parents, or for child-free people), the more it should count in my guess of your status.

What's the best way to calculate this?

For extra credit: what's the best way to calculate this, that is also cheap & easy to do in mysql?