[lomnakkus]

There's usually much more variation between individuals within a group (say, all white people) than between groups (say, white vs. asian people), nevermind individuals from different groups. Judging individuals based on aggregate statistics about their group would be foolish in the extreme. (Not to mention morally dubious, at best.)

Or course it depends on the size of the group, and you can construct artificial groupings to make the above false, but let's just say that all the 'standard' groups are covered: ethnicity, skin colour, gender, sexuality, etc. etc.

[sevenless]

> There's usually much more variation between individuals within a group (say, all white people) than between groups (say, white vs. asian people), nevermind individuals from different groups.

This is statistically illiterate. Let's say you have two sets of random variables X and Y, both from Normal distributions with standard deviation 2, where the mean of X is 0 and the mean of Y is 1. Knowing whether a measurement comes from X or Y will still allow you to make more accurate predictions, even though the within-group variance is larger than the between-group variance. For very large groups, this applies much more so. If you have high-dimensional multivariate data, it is possible to assign individuals to clusters very accurately even if all individual measurements overlap substantially.

See for example http://www.ncbi.nlm.nih.gov/pubmed/12879450

> Not to mention morally dubious, at best.

This is the real issue: the use of certain stereotypes is a political and ethical debate, and those against using stereotypes should stop pretending there are no costs in terms of sub-optimal decision making. There are defensible ethical reasons for being against some types of discrimination based on valid stereotypes.

[iopq]

Let's break it down by gender, a woman is an order of magnitude less likely to mug you than a man. The difference between groups (female or male) is MUCH larger than the difference in the groups (white male vs. black male or white female vs. black female)

[taejo]

> white male vs. black male or white female vs. black female

These are still groups. I (a male) am much less likely to mug anyone (let's say 1 in a million chance this year) than certain individual women. Within the group of men and within the group of women, the "mugging probability" varies from almost zero to almost one.

[iopq]

I have no way of knowing YOUR percentage when I see you in a dark alley. It's safer to just run and have you be offended at me.