And, from skimming the paper, they have literally divided companies into two groups, <35 and >35. It could be that one giant employer (like a hospital system) i

And, from skimming the paper, they have literally divided companies into two groups, <35 and >35. It could be that one giant employer (like a hospital system) is the whole story. A more subtle study would be to look for a discontinuity: expect that firms with 10, 30, 100, 1000 employees will behave differently, but anything which happens sharply at 35 (and changes in this time) can be more confidently attributed to the law. But this is obviously harder to do.