For instance, check out this passage:
[T]he growth rate of a nation's productivity determines the growth rate of its average income. The fundamental relationship between productivity and living standards is simple, but its implications are far-reaching. If productivity is the primary determinant of living standards, other explanations must be of secondary importance. For example, it might be tempting to credit labor unions or minimum-wage laws for the rise in living standards of American workers over the past century. Yet the real hero of American workers is their rising productivity.They sound pretty sure of that, don't they? Productivity is king. If you want more prosperous workers, increase productivity -- all that other stuff is secondary. Well, they are economists, so I'm sure the evidence will prove them righ-
Oh no! Something's gone wrong! Whatever could it be?!
Of course, nothing they've said is false, strictly speaking. As society gets richer, mean incomes go up, and sure enough, that's one kind of average. And real wage increases would be impossible if productivity hadn't increased.
But the textbook authors have conflated mean income with "living standards of American workers," a formulation which contains an unspoken assumption that income growth is distributed evenly. Increasingly, it's not.
In short, their example suggests a number of conclusions that simply aren't borne out by the evidence. And they could have avoided the mistake altogether if they'd avoided the temptation to include an underhanded little jab at unions and worker protections.