Originally Posted by Drip

The chance of failure probably doubles with each skill level. Something along the lines of
1->2 10% failure 90% success
2->3 20% failure 80% success
3->4 40% failure 60% success
4->5 80% failure 20% success

^ This is much more plausible than 50/50. I am 110% sure it's not 50% chance every level and it does get more difficult the higher it is. I just don't know the exact percentage.

I have not had a lvl 1-2 skill fail on me yet... 2-3 seems to work about 80% of the time and 3-4 seems to be closer to 50-50 while 4-5 seems to take at least 5 attempts some times as many as 10 with a few rare 1-2 flukes.