I wanted to post up some probabilities for Runequest: Roleplaying in Glorantha. After I asked on Discord, Tommi generously crunched the numbers for me. You can view the spreadsheet he built (and lots of other game-related stuff) at his blog.
As background, there are two methods of opposed rolls in RQG. The first is to match a character’s Characteristic (STR, DEX, etc.) against the Characteristic of an opposing force (the SIZ of an object they’re trying to move, the POT of a poison they’re trying to resist, etc.)
Characteristics are compared on the Resistance table to find a probability of success, the ‘active’ character rolls, and if they roll under they overcome the resistance of the opposing force and win (lift the boulder, shake off the poison, etc.)
The second method is an Opposed Skill contest. Each character rolls against their Skill independently and checks to see if they achieved a Fumble, Failure, Success, Special Success or Critical. The character with the higher type of success wins, and in case both achieve the same kind of success, the character who rolled higher wins.
The question I wanted to answer by looking at probabilities for Opposed Skill contests was how interchangeable Opposed Skill rolls were with the Resistance Table (and, by extension, how interchangeable Skills are with Characteristics).
Take for example a contest between a Skill of 75% and a Skill of 45%. Would the probability of success for the character with 75% be the same as if we’d converted the Skills into Characteristics (75% / 5 = 15 and 45% / 5 = 9) and compared them on the Resistance Table?
As you can see, the probabilities for Opposed Skill contests are significantly different from the Resistance Table, particularly where the differences are large. The example above (Characteristic of 15 vs Characteristic of 9) yields an 80% probability of success on the Resistance Table, whereas treated as an Opposed Skill roll (75% vs 45%) it yields a 75% probability of success, to say nothing of the added textures of having both participants' rolls output a result (both succeed, both fail, one attains Special Success while one Fumbles, etc.)
I don’t want to speculate much on how these differences impact play. The only thing that seems immediately noteworthy is that a Skill or Passion augment is hugely impactful when used on the Resistance Table--on a regular success or failure, you’re looking at +/- 20%, the equivalent of 4 Characteristics points! And this can easily (if temporarily) push a character over the usual limits of their species maximum.
Augments of Opposed Skill rolls, by comparison, have a smaller impact (a 20% shift on a skill our 75% skill only increases the chance of success by 11% on a regular success or decreases it by 16% on a failure).
I’d love to hear others’ observations of these differing systems, and I’d be especially curious to hear how different tables have used the Resistance Table in play. I have this sense that the Resistance Table can be used to make the immediate terrain (boulders, thickets, waterfalls) interactive and tangible. But I haven’t seen it realized in play yet.