Coldtea: He is saying that there are obvious lurker (or "confounding") variables. It could be that not getting a degree is caused by being poor (or black, etc.

Coldtea: He is saying that there are obvious lurker (or "confounding") variables. It could be that not getting a degree is caused by being poor (or black, etc.) AND that not getting a job is caused by being poor. So not having a degree and not having a job may appear to have a high correlation when in fact they are both driven by a third variable (socio-economic/race/etc.). Look up Simpson's paradox.