Should you use Fate Dice for Your Reaction Rolls?

tl:dr; Not something I would do on a whim. It's swingy, but it might make sense for you.You cannot reasonably adjust for charisma though!

Introduction

One of my favorite things about retro-adventure games is the random reaction roll. When the party encounters a creature, and you're not sure how the creature should behave, you can roll 2d6, adjust the result based the charisma of the party, and compare the total to a chart. The chart defines a range of possibilities from outright hostility to immediate friendship. In Old-School Essentials (the system I'm currently using) that chart looks like this:

The reaction roll chart from Old-School Essentials

This system is basic, but surprisingly elegant. 2d6 rolls on a bell curve, meaning that you're much less likely to get "critical reactions" (2 or 12) than you are to get neutral outcomes - where savvy role-playing can win you an ally or avoid a combat.

The annoying part about the 2d6 reaction roll is that it takes some time to learn. You're going to have to run a few sessions and make lots of rolls before it sticks in your head that a 9 is "indifference" versus an 8 which is "neutral". And it isn't entirely clear why the ranges where broken down the way that they were. One way we might make this process easier is...

Using Fate Dice Instead

Basically some d3 that are numbered from -1 to 1 twice

The set-up is pretty simple. Instead of rolling 2d6 for the reaction roll, we'll roll 2dF; treating the minus sides as -1, the blank sides as 0, and the plus sides as +1. With that in mind, we get the following possible results:

-2: Hostile
-1: Unfriendly
 0: Neutral
 1: Indifferent
 2: Helpful

Before we go any further, I believe this is much more intuitive. Instead of adding numbers and comparing them against some range on a chart, all we have to do is count plusses and minuses. The ranges themselves also make a bit more logical sense. 0 is the neutral ground, and each step on the reaction scale is an equivalent step on the number scale.

Okay, so plugging the rolls into AnyDice we get the following probabilities:

2d6 vs. 2dF

Interesting...So, the first thing that jumps out is that using fate dice give you a combined 22.22% chance of getting a critical reaction (attack or help on sight), versus a combined 5.56% chance using 2d6.That's pretty extreme. To put that into perspective, the standard 2d6 roll will give you a critical reaction 1-in-20 encounters or so. Using fate dice, that increases to almost 1-in-4. 

The vast majority of that difference is coming from the neutral result, which is 11 percentage points less likely using the fate roll. The chances for the middling reactions (Hostile or Indifferent) go down, but only by ~2 percentage points each. I don't think that makes a huge difference.

Putting that all together? Using fate dice will lead to much more swing in reaction rolls. You're significantly more likely for encounters to start with either violence or friendship, and noticeably less likely to get a neutral result. I'm not convinced that's something I want to introduce all of the time. However, I could see that being very useful in a faerie realm or the like - a place where moods can swing wildly for reasons that aren't necessarily obvious to normal people.

However, there's one major flaw to this system: Charisma adjustments DESTROY your results.

The Impact of Charisma

Because each reaction in a 2dF chart contains only a single number, every point of adjustment propels the reaction a full category up or down. That means if you're using a system with static bonuses or penalties for charisma scores, your "random" reaction becomes shockingly predictable.

2d6 vs 2dF - Including positive charisma adjustments

Using 2d6, a player with a +1 charisma bonus will get the best possible result 8.33% of the time (1-in-12 encounters) and will get the neutral result 41.67% of the time (a little less than 3-in-7 encounters). Using 2dF the chance for the best result soars to a staggering 33% (1-in-3 encounters!) And the neutral result plummets to 22.22% (2-in-9 encounters).

A player with a +2 bonus (the maximum in OSE) has a 16.67% chance to get the best possible result (1-in-6 encounters) and has an 8.33% chance to get a hostile reaction (1-in-12 encounters) using 2d6. But using 2dF, that character has a 66% chance of getting the best possible result (2-out-of-3 encounters) and, crucially, cannot get a result worse than neutral. 

The results for charisma penalties have the same percentage chances, just in the opposite direction. Players with a -1 charisma penalty have a 1-in-3 chance to be attacked on sight, and players with a -2 penalty have a 2-in-3 chance to be attacked. And the characters with the worst charisma cannot get a result better than neutral.

2d6 vs 2dF - Including negative charisma adjustments

I mean, if you're sick of players dumping charisma, this will change their minds really quick. The reaction roll swings so wildly towards the extremes that it's almost comical. It won't take long for players realize that they're consistently getting positive or negative reactions, and once they do, all the drama of meeting a new creature will evaporate. Predictability is the death of tension, especially when 2 out of every 3 people you meet predictably want to bludgeon you to death with a stick on sight.

Conclusions

Based on the probabilities above, I don't think that using 2dF for reactions is a good idea if you also use charisma adjustments. The 2d6 table has multiple values that map to the same reaction. That allows charisma adjustments to nudge reactions gently in one direction or the other. 2dF has only a single value map to each reaction. That's easier to memorize and understand, but it means that any modifiers swing the results violently in one direction.

That said, a straight 2dF roll with no adjustments gives you a simplified reaction chart that seems perfectly usable. I would consider using 2dF on its own for a one-shot, or when trying to model a whimsical, cartoon-like world where tempers can change rapidly for the smallest reasons. But for a standard dungeon crawl, I think I'm going to keep using the 2d6 table.

Pros to 2dF:

• Simpler to see at a glance: you just have to count plusses and minuses!
• Don't need to memorize ranges or refer to charts

Cons for 2dF:

• A lot more critical reactions means less opportunity to role-play negotiations
• Randomness breaks down if you add any static modifiers
• Fewer neutral results