[pauljz]

Regarding the US to China route, this is more complicated than it would seem. While getting a visa is probably the easiest solution, there are some situations where it is not required. An example being that you can enter visa-free into Shanghai for 48 hours if you're in transit through the airport.

There are some other odd visa-on-arrival schemes available for places like Shenzhen if you're arriving via Hong Kong (although I think this isn't available to US citizens, only to Australians, and a few other nationalities). I wouldn't be surprised if this is true of some of the other Special Economic Zones as well (e.g. Hainan island). It can be really hard to find this information though, even through official channels.

So this kind of confusion is what the author is up against when collecting data.