d20 Ways to Nerd Out About Partial Success

Prepare for some navel-gazing and boring math that probably doesn’t even feel relevant at most people’s tables. It’s mostly for my own reference down the line, but maybe you’ll find something you relate to here.

I love non-binary dice rolling systems. Even just adding a single option between success and failure—“yes, you succeed, but…”—adds a lot to a system for me. I’ve enjoyed this the most in Apocalypse World and its “Powered by the Apocalypse” (PbtA) descendants, which have a really satisfying set of ranges for roll results: fail when you roll low (≤ 6, 41⅔ percent), succeed at a cost when you roll in the middle (7–9, 41⅔ percent), and succeed without cost when you roll high (≥ 10, 16⅔ percent). I’d probably run the vast majority of games in my future in that system from here on out if it weren’t for just two niggling issues: Sometimes, I like playing games where trickier problems are reflected by tougher odds, and oftentimes—especially when playing old school, D&Dish games—I really miss rolling 20-sided dice.

I’ve read a few approaches to graduated results on a d20, including Talislanta (too many result ranges for my taste), Forthright (which is good, but not really the numerical breakdown I’d want), and a blog post on Questing Beast (also good, but requires more dice than I want to roll at once unless I’m using “advantage” rules). Necropraxis offers one I do think I’d like to try, though, because it’s just so easy to remember:

  • FAIL on single-digit numbers, 1–9 (45%)
  • PARTIAL SUCCESS up to double the fail range, 10–18 (45%)
  • FULL SUCCESS on 19–20 (10%)

The symmetry with Apocalypse World’s system, where failure and success at a cost are equally likely, is a nice touch.

Here’s where I start getting really picky, though: What I’d really love is a d20 to-hit system that requires you to roll high, allows for partial/mixed success, and also makes it easy to run with all player-facing rolls.

(Maybe don’t worry about why I want this. I just do. But, okay, fine, I admit: I also have an unreasonable hope that it would somehow make it easy for me to use it to run both my many Dungeon World adventures on hand and my many OSR adventures on hand.)

The above ranges from that Necropraxis post can definitely work for this purpose, but I do see one potential hitch: If rolling a partial success in combat is likely to be interpreted as “you and your enemy both damage each other” (and that seems the most likely way I’d run it, and in keeping with how melee combat works in PbtA games like Dungeon World), then the odds are really brutal for player characters. Yes, the odds of “success” are ostensibly in PCs’ favor, but the odds of having the option to deal damage are in their enemies’ favor – enemies deal their damage on any roll up to 18 (85% of the time), but PCs only deal damage on a 10 and up (55% of the time). And since a +1, 2, or 3 attack bonus doesn’t go as far on a d20 as it does on 2d6, I feel the odds actually kind of matter more.

For these reasons, I wonder if I’d be better served by odds like…

  • FAIL on 1–5 (25%)
  • PARTIAL SUCCESS over 5, up to 15 (50%)
  • FULL SUCCESS over 15 (25%)

With this approach, you end up with even odds that an “average” PC (+0 to attack) and any given enemy both injure each other, and an equivalent chance between them that one damages the other but comes out unscathed. It really makes some degree of success more likely in general (up to 75% from 55%), and also makes partial success a slightly more likely result too (up to 50% from 45%). I may try this distribution someday, too.

All of that said, this is still missing one thing I pined for earlier: a comparably simple and satisfying way to account for variable difficulty by target numbers, like enemy armor class. The above rolling systems might work fine to try running some Dungeon World scenario with a bit more of a D&Dish feel, but I’d also love to be able to just slot a different rolling mechanic into an OSR module, B/X to-hit tables be damned.

The quickest conversion I can think of offhand would be use a variant on a system I’ve been tinkering with for some time I call “Greyscale” (so named because it was originally inspired by both Black Hack and Whitehack rolling mechanics, but with variable results). The way I first conceptualized (the roll-over version of) it was with a difficulty between 1 and 10, and a partial success range extending to that difficulty + 10. In other words…

  • FAIL if not over X
  • PARTIAL SUCCESS if over X, up to X + 10
  • FULL SUCCESS if over X + 10

At 8 or 9 difficulty, with that approach, you would have similar odds to the Necropraxis ranges noted above; at 5 difficulty, you have the exact (over 5, or over 15) distribution I proposed just after. The “X” could potentially be enemy HD, or potentially spell level for a slot-less casting system based around ability checks. Or, alternatively, use the target’s AC as the upper range to attack enemies, and define the lower end of the partial success range as AC minus 10. And for anything that doesn’t have a set target number, I’d be comfortable just telling the GM to go with 5.

I imagine there may be better ways than “the quickest conversion I can think of offhand,” but I suppose something like this is my working plan to try sometime—at least until I see some other approach that gets me to nerd out even more. And sure, the world has more than enough dice systems already, but sometimes, thinking way too hard about trivial details is precisely what I want from this hobby.

This post originally appeared on Google+ on April 28, 2018.

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