It looks like that data comes from the Bureau of Labor Statistics. I'm going to go ahead and guess that they don't include undocumented workers. A lot of those

It looks like that data comes from the Bureau of Labor Statistics. I'm going to go ahead and guess that they don't include undocumented workers. A lot of those construction jobs are probably only looking at union work. The author says as much below. I don't know what it's like for electricians or carpenters, but the concrete business has a lot of undocumented workers, especially on non-government jobs. And even when they are somewhat legit they don't typically join the unions or get tagged as 'cement masons' (finishers) even though a lot of the time they do finishing work as well as the general labor.