Hi Jean,
In response to your question:
"Can we conceive of it both ways? Am I missing a nuance between examples that makes it confounding in one situation and selection bias in another?"
It has taken a while to respond bc I and the TA's are not sure we understand your question or point of confusion.
However, upon further reflection, I think I might get where you are coming from.
Please note - In the paper for HW1, the outcome was incident prostate cancer in generally healthy cohort of men. In HW 2, where we raised the issue of prevalent user bias/selection bias, please note the difference in study populations and outcomes. Both studies have a vulnerability for confounding. The HW2 study has more of concern for prevalent user bias leading to a selection bias, because of the study population of interest (i.e., cohort of men with prostate cancer, examining progression). Can confounding and selection bias be happening simultaneously in the 2nd study, I think Yes, and it's harder to deal with the latter.
Hope that helps,
JMC