This is called a conditional value list. Here are some links that can help. For your specific case with "Supervisor" you may find the "option 1" method in the tutorial link an effective way to go:
Forum Tutorial: Custom Value List?
Knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list
Hierarchical Conditional Value lists: Conditional Value List Question
Feel free to post follow up questions here if those links aren't enough to get your value lists working correctly.
Thanks for the super quick response Phill, the option 1 seems to fill my needs. What is very nice is that the list exclues "empty" values.
It's not intuitive, at least for an old SQL junkie, but it works nicely and, I think, quickly.
In FileMaker, developers familiar with SQL often reach for their familiar SQL "tools" only to find that FileMaker uses a very different method to achieve the same end. On the other hand, Execute SQL is letting us do a little but of SQL querying in FileMaker 12 that was not possible in older versions.
What seems to be important to me is to "keep an open mind." I have to remember that there is more than one way to skin a cat, and the more I use Filemaker, the better I'm getting at guessing the "Filemake way."