[tierack]

Typically, this sort of pattern means that "low income" includes low income students across all races, that's the only selection criterion for that group. The "white" group will include all white students (including low-income), and the "black" group will include all black students (including high-income). Nothing nefarious.

[jff]

That's what I was hoping, and that's how I'd structure it if I insisted on including a "low income" group when everything else was racially divided.

I hope that the authors did it as you described. Would I be flabbergasted if they pulled some shaky crap? No, probably not, but to quote Mulder's poster, I want to believe.