[gravypod]

1. Focus on a simplified version of a real problem you had to solve at your current job (MIN(arbitrary))

2. Needs to be explainable in 1 page with example data (MIN(completely))

3. Needs to not require any complex interaction. No web request, no complex libraries, just a unit testable piece of code. (MAX(impl speed))

4. Make it fun and something you and all your coworkers could easily do.

This has been my approach and it's been a good filter. If it's done before the interview have them come in and add a feature. Make sure they bring a machine with their prefered IDE / editor to maximize similarity to their real programming flow. Offer to do something else while they code so they don't feel "watched"