Location (within a single country) is Irrelevant for Usability Studies -Jakob Nielsen

As long as you're testing within a single country, there's no reason to expend resources traveling to multiple cities and conducting the same usability study again and again. You'll simply observe the same behaviors repeatedly, and learn nothing new.

This conclusion -- that the test location doesn't matter -- is different than the usual lesson from market research, where you find different results in different regions of the country.

Why does usability differ from market research when it comes to the number of required study locations? Because with usability, we test behavior, not opinion. Further, we test that behavior with a defined artifact (i.e., a specific user interface).

…In most cases, differences are due to diversity in the users' circumstances, not to geographical variation. It's therefore best to recruit a diverse set of users: some experienced users, some novices…