25% of 100%, vs 100% of X%, means that there's a missing number if we want to seriously compare them...
Agreed.
1) Second Chance is only used on a successful hit. So, 100% to start with. If the attack missed, Second Chance would not be used. It also works against all attack rolls, regardless of defense, so another 100%. With a 50% chance to hit, we have an original 90% chance of a normal hit (11 to 19) and a 10% chance of a critical (20). So the average amount it protects is calculated by:
100% * 100% * (90% normal damage * (50% of a miss - 5% chance of a critical) + 10% critical damage * (50% of a miss + 45% of normal damage))
This works out to an improvement 54.5% of the time and a worsening of the situation 4.5% of the time. 41% of the time, no change. Ironically, one could distill this down to a 50% chance of improvement (for a 50% chance to hit).
These percentages are higher if the player mostly uses the power to minimize a critical, but if he does that, there are times when he won't use it in an encounter at all. Pros and Cons.
2) Shield also is only used on a successful hit. So, 100% to start with. However, Shield works differently from game to game depending on the DM's adjudication:
2a) The DM rules that the PCs do not know anything about the attack.
73% of the time, a monster will use an AC or Reflex attack (on average in the MM).
If an attack occurs with a 50% chance to hit, Shield will nullify the attack 40% of the time (4 numbers out of the 10 possible to hit numbers).
So, 100% * 73% * 40% = 29.2%
2b) The DM rules that the PCs know the type of attack (e.g. Fort or Reflex), but they nothing about the die result or total.
100% * 100% * 40% = 40%
2c) The DM rules that the PCs do not know the type of attack, but rolls the attack die in front of the players.
If the players see an 8 through a 14 or so on the die, there is a very high chance that Shield will work (not all creatures have a 50% chance to hit). If they see a 17, Shield will almost never help (although it would have once in our game). This is also dependent on how high an AC or Reflex the PC Wizard has (higher ones bump up the range some). For the most part, the player has a high chance (80% in my estimation) of guessing correctly if he can see the die roll.
100% * 73% * 80% (a guess) = 58.4%
2d) The DM rules that the PCs know the type of attack and rolls the attack die in front of the players.
100% * 100% * 80% (a guess) = 80%
2e) The DM rules that the PCs know everything (i.e. the total attack roll and the type of the attack).
100% * 100% * 100% = 100%
Granted, 2e is not a true 100%. It is just 100% of the first attack in an encounter for which Shield will definitely work.
But, the comparison of 1 and 2a does show us that worse case scenario is 29.2% for Shield versus 50% for Second Chance. What it does not show is that Shield also can be used for multiple attacks versus one for Second Chance. That ups the 29.2% somewhat, especially in situations where the Wizard has multiple foes. Even if the first use of Shield does not stop an attack, later uses in the round might.
So, doesn't 2d and 2e seem overpowered when compared to 1 considering that Second Chance cannot stop multiple attacks and Shield can?
2a and 2b seem like the most balanced adjudications.
When one also considers how frequently people take Shield as their primary second level utility power, one should consider if 2a or 2b is the more reasonable adjudication. Personally, I think 2a is more balanced, but I can see why people would pick 2b.
2e is ridiculously powerful and useful compared to Second Chance.
Note: We use 2c in our games because the DMs always roll attack dice in front of the players. The game does not feel forced or contrived or fudged if the DM is open about his attack and damage rolls. What happens, happens. No guaranteed PC survival entitlement and limited DM punishment for stupid mistakes.
I consider 2c to be reasonable and the math above pretty much illustrates that. Shield actually works out to be more useful than Second Chance with 2c, even if one changes that 80% guess.
Btw, I am old school. I think telling players nearly everything (like MM's post) is part of our recent entitlement society. People are used to seeing damage bars above foes in mmorpgs and other computer games and think that handing information out like candy is reasonable. I do not. I think there should be some mystery in the game system, or why bother to play DND instead of an mmorpg. Even the monster skill checks are way too generous in the information that can be acquired. IMO.