Schools near state lines perform worse and rules discouraging teachers from moving may be to blame

[Igel]

2. I'd want to see their data.

I'm not saying they're wrong. They sound completely plausible. I teach in Texas, I could be certified in some states but others might be more difficult because certification requirements differ so much.

But I know that many of the states I've lived in have their population centers well away from the state border. If you look at the population near the state borders, it's a small percentage of the total population. Not so true in the NE megalopolis, or in places like Portland, OR or El Paso, TX. But at that point, you start wondering which of the 50 states are in their sample of 33.

At that point you also start wondering about the teachers who wind up in the middle of nowhere teaching. Might be interesting to match state-internal schools with boundary-proximal schools at a given distance from major population centers and see if there's a difference. I.e., check for a state-line versus rural confound.

Esp. since the effect is small.