You roll dice to find out what happens, right?
OK, how do you determine the parameters for “what happens?”
When you roll dice in a tabletop RPG, you’re generally comparing the roll to one of three things:
- A target difficulty number that varies according to either the rules or the GM’s decision during play (as in D&D, where the DM decides that a particular trap requires a Dexterity roll of 12 to disarm, or you roll against a monster’s Armor Class of 15).
- A static number defined by the rules (as in Apocalypse World, where 6 or less is always a miss, 7-9 is a weak hit, and 10+ is a strong hit)
- A character statistic (as in GURPS, where you roll 3d6 and if you roll less than your character’s most relevant attribute, you succeed)
Each of these has different, subtle effects on play.
If you roll against a target difficulty number, someone has to think about all of the situation’s parameters ahead of the roll. This not only informs the roll; it can lead players to define the situation too rigidly. If you decide on everything that goes into the roll, you can then imagine what both success and failure will look like.
Rolling against a static number or a character statistics alleviates this problem. While you do have to decide on the modifiers that may apply, you don’t have to define the problem in as much detail. This frees you to make the roll and then interpret its results.
In other words, using a target difficulty number tends to pre-determine the outcome: You end up defining what success and failure will look like, because you have to take those into account when choosing the difficult number. Using a static number or character attribute simplifies the roll, which lets you think about what success or failure look like after you know whether you succeed or fail.
The latter approach feels to me like a more elegant way to weave story into dice rolling.