Sure there is, all immigrants are people who moved here from other nations. They may have different reasons, times, methods, causes, and there may be different subgroups, etc, but there certainly is a single group of immigrants just like there's a single group of Americans, sheep, and pizzas.
Surely you understood what I was saying but so we don't talk past one another: Yes there is a category of people called an immigrant that one can define. However when you look at groups or even individuals in that category there are certain differences, many times vast and very meaningful differences, between the things in each of those categories. So to treat the whole group the same would be foolhardy; thus the need for a selection process. If you had to purchase 10 sheep from a group of 100 would you not buy the best sheep you could afford or the best pizza you could afford?
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.
