Regular force IAP / BOTP sees three failure types from my experience at CFLRS St. Jean(Which is now one and half years outdated):
Some people arrive inadequately prepared physically and/or mentally. Many factors play in: Personal disposition and compatibility with military life, missed loved ones, anxiety and the somewhat misleading step test as an indicator of fitness all contribute to Voluntary Releases(VRs) in the first two weeks. Some very strong individuals will fail out if they lack the ability to gain and use the support of their fellow candidates. Amazingly, some people can make a platoon of some thirty interesting people into a place of solitude. Don't let it be you.
Anyone can suffer an injury or be called away for extra-military reasons during training. These are a little hard to nail down numerically due to the circumstantial nature. Furthermore, if someone is mentally deficient enough to commit serious offences they will be removed.
Training failures are assessed when the candidate fails a test or graded task twice. At this point a progress review board is convened with three outcomes: Recourse, Release, Retry. A retry seems only possible if a force outside the candidate's control interfered with a task and is more than rare.Example Course IAP - 32 Candidates
Initial Failures (9.375%)
-Two VRs due to 'this is not for me' syndrome
-One candidate encouraged to VR due to inadequate physical fitness
Transient Failures (3.125%)
-One Medical Recourse from something foot related
Training Failures (0%)
TOTAL LOSS of 12.5%
Note: I found IAP to be far less demanding than I had expected. It delivers in the 50 tasks with time to only do 10 category - keeps you busy and fatigued. However, the final tasking seemed to lack realism and did not warrant the micromanaged planning methods expected. In addition, I thought that it may have been more beneficial to cull the herd earlier, rather than later by assessing in a more strict manner. I am told that is what trade specific training is for.
Joy. Example Course BOTP - 28 Candidates
Initial Failures (3.57%)
-One VR due to 'this is not for me' syndrome.
Transient Failures (0.0%)
-Lucky in this department. A girl from another platoon jumped foot first onto a large, rolling rock when disembarking a helicopter and rolled her ankle worse than anything I have ever witnessed. She did get a first class ticket back to the hospital. Resulted in a medial recourse, or so I am told.
Training Failures (3.57%)
-One training failure on final exercise. Resulted in recourse and a subsequent release due to further failures.
TOTAL LOSS of 7.14%
So provided fate does not pay you a visit, you keep your head up and a sense of humour at hand IAP/BOTP will be nothing but good times. Of course, hindsight tends to favour the more pleasant memories.
Feel free to post any other questions or fire me an e-mail if you have any specific queries.