I'm sorry if I step on any toes but as a person with a physics, mathematics and philosophy background I wish to make some commentary upon this guide in the hopes of making the guide as effective as possible since I think the idea of how to handle Iterative Probability is very important in a game that is so reliant upon repetitious rolls of a dice.
1) I think a valuable inclusion would be a discussion about probability especially how to calculate probability and a chart that shows the long term probability of common iterative states.
This can be calculated via: [Desired threshold = Chance success ^ #effects] or via [Log (Desired threshold)/Log (Chance Success) = # effects]
Here is a quick little table I whipped up in Excel in a couple of minutes to give a basic idea (Sorry for the fugly formatting)
Desired Threshold of Failure (%chance to have rolled a failure)
0.75 0.5 0.25 0.1 0.01 0.001
Chance 0.05| 1 1 1 1 2 3
of 0.1| 1 1 1 1 2 3
Success 0.15| 1 1 1 2 3 4
0.2| 1 1 1 2 3 5
0.25| 1 1 1 2 4 5
0.3| 1 1 2 2 4 6
0.35| 1 1 2 3 5 7
0.4| 1 1 2 3 6 8
0.45| 1 1 2 3 6 9
0.5| 1 1 2 4 7 10
0.55| 1 2 3 4 8 12
0.6| 1 2 3 5 10 14
0.65| 1 2 4 6 11 17
0.7| 1 2 4 7 13 20
0.75| 1 3 5 9 17 25
0.8| 2 4 7 11 21 31
0.85| 2 5 9 15 29 43
0.9| 3 7 14 22 44 66
0.95| 6 14 28 45 90 135
We have threshold across the top and chance of success going vertically with the numbers that correspond being the number of iterative attempts that it takes for the probability of failure to cross that threshold.
I think this is very illustrative of why Immunities and rerolls become increasingly important over higher levels because even with a high threshold of 75% and a high success rate of 95% (aka anything but 1) it only takes an average of 6 save or lose effects to cross that threshold. At higher level play one can expect said number of save or lose effects in every combat and thus ways to avoid them become increasingly important.
2) While you did talk about it a bit I think it is important to note the fact that while over the course of a long term of play the chance of failure approaches unity that each individual roll is made in vacuum and is completely discrete from any other roll. Thus while a chart like the one posted above serves as a nice guide it does not in any way mean that if you are making your xth roll that the chance to fail is any different than it has been in any of the previous rolls. This is a common misconception about probability and any post seeking to discuss probability fully should address this, if only for completeness sake.
3) A discussion of the complexity of calculating full-attack schemes versus AC should probably be included. The full-attack schemes of monsters with various natural weapons that all have different attack bonus and damage amounts can become very mathematically tedious to calculate and are extremely unintuitive to nearly anyone who doesn't deal with statistics on a regular basis. This can lead to a real difficulty in properly valuating AC boosts which is a major threshold for players going from the intermediate levels of skill mastery to the more advanced. This is probably most important for DMs who are attempting to gauge monster strength against party defense as monsters with large numbers of inaccurate attacks and monsters with small number of high accuracy attacks deal damage in very different ways which interact with the parties defenses (AC, Miss chance, etc.) in ways that are far more complicated than initial perception.
4) A reiteration of the previous points but I think it really should be included: More maths! You are writing a guide about iterative probability and yet do not include a single formula for calculating probability or any other rigorous mathematical treatment of a subject that is by its very nature defined by such things. Illustrative examples would be really appreciated because while I agree with the basic conclusions of your arguments the logical train used to go from the premises to said conclusions are largely absent. Without the mathematics and other premises to go by it is not possible to evaluate if what you are saying is truly valid or if it just agrees with my own biases. If this is to be a comprehensive guide to IP Proofing then one needs to be able to see the entirety of the thought process for why such a thing is relevant. Otherwise the guide consists entirely of "do as I say because I am right" as opposed to showing the audience why you are correct and thus allowing them to naturally come to agreement. Plus inclusion of further premises and mathematical foundation allows for a better understanding of the nuances of your views and can allow for important corollaries and caveats to arise, which in any subject as complex as CharOp are nearly as important as the basics.
