I have never been called forward for CDT so you might have to wait a long time to refuse one.
I did 4 out of the last 5 at my unit before I left, I was an 'A' candidate for each of them.
I, nor any of the other people who took the test whilst I was there, failed, but then again, we were all old fogies, not a single young chap/chapess to be seen. Have they become properly random, or is the aim to conduct the test and have the least number of failures by targetting those tested.
The mob I'm in now, use the last digit of the service number to identify testees for any given test.