if the incoming data is not random, but is in fact skewed by factors relevant to the data we're looking for, there's very little value in it
Looking at people who donate sperm effectively is a random sample of men unless there's a causal link between sperm count and the desire to be a donor.
Sampling only people who self-select themselves into one specific group (sperm donors) definitely isn’t a random sample. There are obviously correlations between income, geographic area and age with the desire to become a donor.
Don't sperm banks collect all of that relevant information? I would assume they collect at least age, income, and education. Probably some health history as well.
There can easily be a causal link. It’s a self selected group specifically on sexual reproductive strategy. I don’t need to be able to explain the relationship in detail; it’s enough that it’s plausible and that it hasn’t already been studied.
Looking at people who donate sperm effectively is a random sample of men unless there's a causal link between sperm count and the desire to be a donor.