# Background: Dice Probability Tables

Questworlds is Chaosium's rules and prep-lite RPG system. Fundamental to it is the opposed roll, in which a skill is rolled against a resistance, using 'highest rolled value as a tiebreak i both rolls succeed. The produces a table of probabilities that looks like this:

This table skill on the left, and resistance on top. Both are unrolled so 22 is 42. This table has 2 key desirable properties:

- thanks to the tiebreak rule, it almost always (at least 95% of the time) produces a winner on the first try.
- it only *saturates*, i.e. has successive cells with the same value at the extreme edges of the table. These edges representing a two-mastery advantage that can basically never be overcome, even when the best possible D20 result is opposed by the worst.

For Rivers of Sartar, we want to make similar use of opposed rolls, but using a D100 rather than a D20.

RQ:G has (on p142) a comparable system for opposed rolls, with corresponding table:

Unfortunately, compared to the QW table:

- for some pairs of skills and opposition, it produces a draw more than 50% of the time.
- it saturates fairly early, at a difference of only 100% in skill. With several RQ:G spells doubling effective skill, this leads to a situation where a fight with an reasonably elite opponent and a trollkin have the same, albeit small, chance of defeat.

Rivers of Sartar uses a novel D100-based system that, for opposed rolls, has a table like:

This produces a winner more than 99% of the time, and never saturates for any plausible skill value. It works by making two changes to the core RQ dice roll mechanics:

- using a separate d20 to roll the degree of success (critical, special or fumble),rather than using fractional thresholds of the d100 chance of success.
- for skills over 100, changing the numbers required to hit criticals and specials, instead of subtracting from an opponents skills.

The details are described in the next blog post in this series.

Image credit:

Turn2538, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

**Edited by radmonger**

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