Were you born in Europe?  Yes / No

What could be simpler than a question like this? But have a look at some of the answers:

Indeed, Russia is a country that partly lies in Europe and partly in Asia. So neither yes nor no would be appropriate.
The problem with the Ukraine is a little more complicated. Geographically it counts as part of Europe, but politically definitely not.

This shows once again that pretesting your questionnaire is a wise thing to do.
Of course in our foundation year we keep this question to show our students how even seemingly simple things may go wrong. And then we ask them how they want to solve this problem. What would you do?

We have created a separate category "undecided" and specified it to be a missing value in SPSS.

Back to the story of the t-test