2/3 of Americans get the RDA of Vitamin D. Picking a specific outlier case for your control group wouldn't make a lot of sense for a scientific study (not to mention, as others have pointed out in this thread, would be difficult or unethical.) If you are a vegan living in a low-sunlight location, look into supplementation.

So you're saying 1/3 of Americans don't get the RDA of Vitamin D, yet calling 33% of cases "an outlier"

I'm not vegan and I actually take Vitamin D3 supplements, but wanted to point out the flaw in assuming that everyone is in the same bucket as me.

I'm saying that probably a very tiny percentage of Americans get 0 IU. Which makes 0 IU a very poor choice of a baseline. Since you need some concrete amount to use for your control group, what would you propose using?