A little math question about the difference between modifying dice results vs. modifying target numbers

[peterb]

In one of the VTT's on the market you can create custom modifications to dice rolls. It could be anything, like say the modification to Move Quietly. You could create a button that modifies the next roll and changes the dice result. For example you could add 25 to the roll to make it more difficult to succeed (I know, it's not so intuitive) when sneaking around in scale armor. 

I would rather modify the skill level, but that is not how this particular feature works. But I've been told it doesn't matter. Now, my math skills is not that great when it comes to probabilities, but my gut feeling tells me that modifying the dice result on the one hand and modifying the skill level on the other hand would not yield the same result when it comes to the probability of getting a critical success, a special success or a fumble, am I right?

Would it make any difference as far as criticals, specials and fumbles (from a math point of view) if you add, for example 15 percentiles, to the dice result instead of the skill level (target number)?

Thanks for any insights.

 

[Nick Brooke]

I've looked into doing this for the Astral VTT (only), and it seems to be horribly difficult to implement special results as a % of skill level. These VTTs are mostly designed around D&D, so they can handle one number being a Critical, but other than that? No chance, mate.

You're right that adding +15 to the dice roll won't give you the same range of results as subtracting -15 from the skill level. For one thing, critical successes become basically impossible (let's say you rolled an 01: now add +15, making 16, so that's a special success if your skill is 78%+), and fumbles much more likely (at least a 16% chance).

[peterb]

Thanks Nick, that's was I suspected. As for Astral, it does seems to be a bit primitive compared to the competition. The VTT I use can display the result and success level of skill rolls. But it's also geared toward D&D, which is why its modification feature works a s it does (modifying dice roll results).

[Psullie]

The RQG sheet on Roll20 allows you to roll a skill with Modifiers.

[Nick Brooke]

It would be easy to use Astral just as a shared map, maybe with “health bars” to display current Hit Points, Magic Points & Rune Points for each character, but I looked at what it’d take to implement BRP skill rolls and modifiers (inc. spells) and recoiled in horror.