A little over a year ago (maybe longer?) there was an active thread on this issue. I did some research, and had some people send me some data from their own runs.
The data consisted of what skill level was being attempted, how many attempts it took to gain the skill level, and if the combine that actually gave the skillup was a success or a failure. The results were weighted by Ishwar's success formula (the one in the FAQ) to balance the fact that at low skill levels the chances of a success are lower and at higher skill levels the chances approach 95%.
My data paralleled the other data that folks sent me. It showed a pretty consisted 50% bias toward getting a skillup on a successful combine. There was one troubling trade though. The data for pottery showed an actual bias toward getting a skillup from a failed combine. This was consistent for two independent sets of pottery data from 0 to 150 or so. This was before the patch that changed some of the skillup rates though. The behavior of pottery skillups may have changed since then.
One example I specifically remember is that I leveled two characters with equal stats to 102 in Jewelcraft. For on character I made Gold-Amber combines, and for the other I used the next recipe to trivial. The one that made the next trivial combine took 20% fewer combines to get to skill 102 than the one that made the Gold-Amber combine. This was consistent with about a 50% better chance to get a skillup on a successful combine as on a failed one.
If anyone really wants to see it I can try to dig the data up and post it somewhere.
Boleslav Forgehammer (the statistics freak)
Paladin of Brell in his 60th Campaign
E'ci - Destiny Awaits
The data consisted of what skill level was being attempted, how many attempts it took to gain the skill level, and if the combine that actually gave the skillup was a success or a failure. The results were weighted by Ishwar's success formula (the one in the FAQ) to balance the fact that at low skill levels the chances of a success are lower and at higher skill levels the chances approach 95%.
My data paralleled the other data that folks sent me. It showed a pretty consisted 50% bias toward getting a skillup on a successful combine. There was one troubling trade though. The data for pottery showed an actual bias toward getting a skillup from a failed combine. This was consistent for two independent sets of pottery data from 0 to 150 or so. This was before the patch that changed some of the skillup rates though. The behavior of pottery skillups may have changed since then.
One example I specifically remember is that I leveled two characters with equal stats to 102 in Jewelcraft. For on character I made Gold-Amber combines, and for the other I used the next recipe to trivial. The one that made the next trivial combine took 20% fewer combines to get to skill 102 than the one that made the Gold-Amber combine. This was consistent with about a 50% better chance to get a skillup on a successful combine as on a failed one.
If anyone really wants to see it I can try to dig the data up and post it somewhere.
Boleslav Forgehammer (the statistics freak)
Paladin of Brell in his 60th Campaign
E'ci - Destiny Awaits

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