There is one other analysis that would, I suspect, adjust the OP’s conclusions: age. Hypothesis: sitters vs standers, and other measures of the quality of the job (danger, flexibility…) correlate substantially with the age of the worker. As you go from your teens to 20s to 30s and beyond you tend on average to get better jobs. It’s not absolute, but I bet it’s a very strong trend, perhaps stronger than racial factors. That’s a hypothesis I wish this analysis examined.