We experimented quite a bit. Since it was relatively inexpensive to administer (almost anyone could ramp up to conducting it), we were able to test it in a variety of scenarios (even for intern candidates). It ended up being _after_ the initial phone screen, but early into the on-site. There were several other questions that tested other skills (system design, algorithms knowledge, reasoning about concurrency, etc.). Every engineering candidate did the question, regardless of seniority, at least during my tenure.