VeryAngryFelid wrote:Oh, I think I know where the confusion happened.
First character blocks 20% attacks and dodges 0.8*0.3=24% attacks so it blocks/dodges 44% attacks which is roughly 46% (44/30*100%-100%) improvement.
Second character blocks 20% attacks and dodges 0.8*0.7=56% attacks so it blocks/dodges 76% attacks which is roughly 9% (76/70*100%-100%) improvement
I should have written it something like that:
First character blocks 20% attacks and dodges 0.8*0.3=24% attacks so it blocks/dodges 44% attacks which is 14% improvement (44-30).
Second character blocks 20% attacks and dodges 0.8*0.7=56% attacks so it blocks/dodges 76% attacks which is 6% improvement (76 - 70).
Edit. But then relative measure is a bit useful because there is some difference between going from 10% to 30% and from 70% to 90%, that's why I did use relative measures.
So this actually is super weird, again while you're doing the math, you're coming at this backwards, while technically there's nothing wrong with the math here, the problem is that you and planttissue are using a completely different scale and are proving completely different things, but calling them by the same name.
What this chunk of math actually proves is that
if you already have really good defense, you might have enough defense that you don't need more which is fine for what it's worth, but doesn't really address the notion of improvement, also if you have a pre-defined notion of how much defense is good enough that you don't need more, this would let you concretely express how good your overall defense is.
Planttissue is saying "You can be in two states to start with, either you have enough defense or you do not, if you do, then obviously you should not get more defense, if you do, then *it doesn't matter how much defense you have*, from a desirability standpoint, the bonus provided from a shield is always the same improvement over your current condition*
To put this into a more step-by-step solution:
1. Consider Two characters fighting two monsters Character A (CA), fighting Monster A (MA) and Character B (CB) fighting Monster B (MB)
2. CA evades 70% of all attacks, CB evades 0%
3. CA and CB have the same number of hit points, AC, GMDR, and identical offense
4. MA does 333.33% of MB's damage, such that the amount of damage incoming after everything to CA and CB are the same, (333.33% damage * 70% evaded is roughly 100%)
5. CA and CB both decide that this is more damage than is acceptable.
6. Consider the impact of adding a shield that blocks 20% of incoming attacks to either or both CA and CB, the increase in the absolute reduction in the number of points of damage taken is identical, and since the other values are the same their ability to survive combat is identically impacted.
That CA's potential points of damage if they didn't have the 70% evasion would be much much higher, and that the 20% block provided doesn't do anything about the already-evaded attacks doesn't make any difference at all to CA's ability to survive, they have the 70% evasion, and their incoming damage is what it is, and 20% block helps them exactly the same amount as it does for CB.
So the point is that *even though CA already blocks 70% of the incoming damage* they don't care, they have already decided that's too much, the fact that the absolute improvement is less than the absolute improvement in defense for CB is wholly irrelevant, all that matters is "what improvement would I see by adding a shield to my existing condition" which is *not* effected by how much EV/HP/AC/offense they have.
The question that *is* influenced by how much EV/HP/AC/Offense you already have is "Do I already have enough combat power" That is to say, if CA (and by implication CB) have enough defense for the monsters they face now. However since crawl is a progression, and you can never be too safe, and you can never reach 100% protection (you can only get closer and closer incrementally) and there's always more things to attack you, you can never actually say in practice "I have enough combat power that I cannot be made safer." This method is only useful if you want to compare one measure of "making you safer" vs another measure, so this *is* a useful measure if you want to compare something like "I would like to get this 20% shield block, but it will reduce my offense by 30% to switch to a one-handed weapon" for example, which isn't an unreasonable thing to want to measure, but it's not actually a measure of the *effectiveness* of the SH itself, it's a way to compare the relative effectiveness's of two parts of your combat effectiveness's (Which is only meaningful if you have to sacrifice one to get the other)
So ultimately this comes down to which question you ask, if the question is "Is this shield worth sacrificing some other part of my combat effectiveness, like offense, or additional XP that would go elsewhere in my build" then the measure being used by VAF has some value (You also need to calculate the cost half of the cost/benefit, which isn't mentioned here, but it's at least a start), if the question is "How much more effective will I be in combat (compared to how I am now) if I add this shield, irrespective of any costs" then it's not a useful measure at all, and you want to go with Planttissue/Berder's measure.
This disagreement is entirely predicated on two different answers to two entirely different questions.