Dice step mechanics

So many cool ideas...

A dice step mechanic is one in which modifiers increase or decrease the size of the die rather add a fixed value (here's a somewhat incomplete discussion). Instead of rolling d6+1, you would roll d8, etc. The mechanic hasn't been used as much in RPG's as the fixed value modifiers has, although Earthdawn used it throughout and it shows up in DCC as well (there are useful discussions of the Earthdawn system in a RPG Stack Exchange thread and its links, but be aware that the Earthdawn system also uses exploding dice). Perhaps one reason it hasn't been used much is that to use it correctly, you ideally need a bunch of weird dice sizes like d14 and d16 which aren't as widely available as other RPG dice.

If you have these weird dice or are willing to simulate them by rolling a d20 and rerolling invalid results, then dice steps and fixed modifiers behave quite similarly. The average result on d6+1 is a 4.5, as is the average result on a d8, so fixed modifiers can be replicated by dice steps which increase or decrease the number of sides by two.

Dice steps produce a wider range of results though: neither 1 nor 8 are possible with d6+1, and so they introduce more swinginess (technically, higher variance) to the results. They are also problematic for systems where the highest and lowest roll mean something special, such as a critical success or failure. An increase in the die step reduces the probability of the critical result, which is probably undesirable.

It's purple too!

A dice step system also anchors the lowest possible at 1, but the highest outcome is exceptionally dependent on the modifier. Since the highest outcome possible moves around so much depending on the modifier, it is more difficult to implement a reasonable success / failure check using die steps because the probability of rolling more than x changes dramatically. I came up with a skill system that does OK, and Earthdawn finessed this problem through exploding dice, but it make dice steps more complicated to use for binary outcomes like success / failure.

Where dice steps tend to perform better and are easier to use in system design are in rolls for the magnitude of an effect, such as damage rolls or how long a spell lasts. Whether they offer an advantage over a fixed modifier depends on how they are used, but I think they have the potential to work better in some situations.

For example, consider a str 18 fighter (+3 modifier to damage) with a dagger (d4). With a fixed modifier the fighter is guaranteed to do 4 damage, even on a roll of 1. With a dice-step based damage roll, the fighter's player rolls d10, which might better encompass simultaneously the weakness of the weapon and the brutal strength of the warrior.

Even better, give daggers 2d6 keep lowest roll damage. With a fixed modifier, the 2d6 keep lowest choice, aimed at modeling the weakness of the weapon, reduces average damage a little bit. Average damage for str 18 fighter about 5.7, and about half of rolls result in damage 7 or higher. With a dice-step system, the fighter's player would roll 2d12 keep lowest, leading to average damage 4.5, half of damage rolls 4 or less, but still the very rare prospect of 12 damage. To me, the dice-step damage distribution feels nicer. (It also works with two-handed weapons -- a two-handed sword might be 2d8 keep higher -- reliable and better for high-strength characters).

All of the above considerations are math or system considerations, but there is also the tactile / weird aspect of dice steps. For me, rolling strange dice is fun, but I imagine it might irritate other people. I also suspect hunting around for different dice might slow the game down a little, although this problem could be mitigated by having modifiers that don't change too often and that cause dice to stay in the d6 to d12 range for most rolls.

(This many years old blog post captures some of the charm of Earthdawn).