Yes, but if that superior work ethic is a trait more frequently expressed in immigrants than natives, then that's important, and that's the distinction being ma

Yes, but if that superior work ethic is a trait more frequently expressed in immigrants than natives, then that's important, and that's the distinction being made here. They didn't look for the best immigrants for this study, this was a study of random immigrants. The entire point is to show how this group called "immigrants" is different, not it's subgroups. The selection criteria itself was immigrant vs non-immigrant. you're saying that's not valid because it should have been more narrow, but it wasn't invalid because they were explicitly looking at the group as a whole.