It’s a nationally representative sample of schools involving 90,000 adolescents who were interviewed in school and 15,000 who were interviewed at home—so it’s big. In more than 100 schools, every single student was surveyed. It’s been conducted in waves starting in 1994-1995. Many of the same 15,000 students interviewed at home have been re-interviewed with each wave. The fourth wave was completed in 2008. By then, the kids first interviewed in 1994 had become young adults, so we can track individuals over a long period of time.

Here’s what makes the data really special: In previous research, I could ask if you have any friends of a different race. That question might prompt you to think really hard about anyone you can claim to know who is a different race. You might think, “Oh yeah, I know this guy who’s Asian or black and I talked to him once last year.” It makes us all stretch a bit to find someone who fits that category.

This data is different because the kids were asked to nominate up to 10 friends, 5 of their sex and 5 of the opposite sex. They write down the names. Every student in over 100 schools did this. We can link their lists and analyze all sorts of things. We can look at reciprocity. Kid A nominated Kid B, but did Kid B nominate Kid A? You can do a lot of interesting things with it.