Using Percentile_Disc to compute Median/Interquartiles
Median functions vary in a few ways, particularly when there's an even number of items.

I've blogged about medians, Aaron Bertrand has, Paul White has... I'm in good company. :) I'd recommend you find Aaron's material, which shows better ways to do median than PERCENTILE_CONT.

PERCENTILE_DISC will only choose a value from the list. This is fine if you have an odd number of values, but what if your list is (1, 2, 3, 4)? And then, do you choose 2 or 3? It's valid, but you should define what you need.

If you go with PERCENTILE_CONT, the answer is 2.5.

But if your set is (1, 2, 3, 3) then some would argue that your answer should be weighted by the two threes. 

So you should make sure that you've defined what you mean by median. :)