Well these are the pass rate figures CTC claimed in the opening speech when I went there for Stage 2:
Stage 1: 33%
Stage 2: 40%
Stage 3: 40%
Stage 4: ~75%
If you work all these odds through you get about 3.9%...if you dont include the stage 4 its around 5% so this is where I get those numbers from. As for group assessment Ive heard they want to see team work and no one dominating the group