Because of its nature of being set entirely underground, the Underdark can easily be added to any other world setting.

I think it's a requirement of the world setting, and a healthy one.

This form of external validity has to do with the degree to which a set of research findings applies to real world settings or contexts.

His "alternative medicines, involving ever more government intervention, are highly dubious in many real world settings."

Non-standard races and an original world setting, using skill rolls under stats on 1d30.

Such compliance cannot be guaranteed in real world settings.

The world setting is Earth, but in a historical-mythical setting of thousands of years ago.

Self-handicapping has been observed in both laboratory and real world settings.

A microsociety learning environment enables students to apply classroom knowledge to real world settings,for example, starting a business, making laws, and paying bills, etc.

Stand-up role playing in the real world setting, including most military training (much of which, though, takes a behaviourist approach)