Star Wars: Age of Rebellion – A Deep Dive on Dice Probabilities

A little while ago, I gave Star Wars: Edge of the Empire a reasonably positive review. After another year of playing the game, it’s time to check in again see how it’s faring, and to try to pass along some playing tips for dealing with some of the game’s quirks. If you haven’t played Star Wars: Edge of the Empire or Age of Rebellion (they’re exactly the same game system), you might want to catch up by reading that review – this will go pretty deep into the game’s probabilities and what they mean for actual play.

The dice in Star Wars: Edge of the Empire, image courtesy FFG. Click for product page.

The central, most intriguing, and most opaque idea in the game is the set of customized dice that are used to form dice pools for task resolution. At their most basic, they are simple and elegant: just add positive ability (green 8-sided) and proficiency (yellow 12-sided) dice for your level of skill, negative purple 8-sided difficulty dice for the difficulty level negative red 12-siders for reasons, and then even some more dice (positive blue boosts and negative black  setbacks, both 6-siders) for situational modifiers (cover, time pressure, assistance from an ally), grab them all, and roll them. Net out success and failure icons, and advantage and threat icons. What you’ve got left is the result. If you end up with at least one success, you succeed. Remaining threat or advantage may give an additional bonus or penalty. It’s a pretty cool idea and rolling lots of dice is fun. However, as I played more, I kept noticing unexpected quirks in the results.

The big thing was that critical hits (which generally require a successful combat check with three advantages) seemed exceptionally rare, and even weapons special powers were hard to activate (they generally require successful check with two advantages). A little bit of analysis of the dice and this made sense. The dice faces never have more than two symbols and rarely show both success and advantage. Good dice only have good faces, and bad dice only have bad faces. So canceling out all the failures and all the threats while having surplus successes and advantages is obviously rare. While there are in theory four different possible “quadrants” the result of a dice roll could fall into (success or failure with advantage or threat), in practice two of them (success with threat and failure with advantage) are predominant, with critical success and critical failure seeming quite rare unless you have a big imbalance between good and bad dice in the pool.

Another odd thing was that it seemed like having high characteristics (Agility, Perception, Brawn, etc.) was a much bigger deal than actually having ranks in skills. A skilled character with a lower characteristic (so, say, a character with Piloting 2 and Agility 2) did not seem to do as well as a character with a slightly higher characteristic (so, no Piloting skill but Agility 3).

Then finally, because nobody really had any idea what the magnitude of the effects of adding various dice were, it made it hard to know what to do with the Destiny Pool, or how to rate the importance of boost and setback dice, or how to wisely spend advantages in combat. The rules seemed to imply that a boost/setback die was a lesser effect while upgrading a die (for example, turning green ability die into a yellow proficiency die) was a bigger deal. But is this really true? It didn’t feel like it.

Now, a little bit (or even a lot) of opacity in the odds is not a bad thing. In fact, a major appeal of the Star Wars system is that dice pools are so intuitive to construct but the probability curves they produce are so complicated as to be essentially incalculable.  That said, we still need to understand some basic things to play the game well. And the only way to get there seemed to be to run some Monte Carlo simulations and see what the numbers looked like. So that’s what I did. Knowing these details definitely improved my game, but they also raised some real questions about whether the designer understood how the dice pool actually works in practice, and if there are fundamental aspects of the game system that need to be re-calibrated.

The dice pool is extremely flexible and it can generate a huge range of probability curves. So I focused on a just few questions: how common are critical successes? What are the quantitative impacts on success and critical success of adding the various different dice to the pool? How big a deal are the challenge dice? To do this, I looked primarily at a few common cases:

• A difficulty 2 check, which is a typical ranged combat check at medium range or when shooting at a similarly-sized ship
• A check made by a moderately-skilled entry-level character, with two yellow and one green dice
• A highly skilled character rolling one green and 3 yellow dice

For a look at a sampling of the numbers generated, you can check out this Google sheet. I’ve focussed on positive results, because they get the most attention from the system. The SUC+2 or SUC+3 columns give the percent chance of a successful check with at least that many advantages. I didn’t go deep, running stats for 2 or 3 or more boost, setback, or challenge dice, but this should give you a feel for how they work. You could really go crazy with the data, but the visualization problems get out of hand very quickly. Also, you need to run a surprisingly large number of iterations to get the results to converge. The numbers for the very large dice pools are not exact.

First a few general observations:

• If you’re trying to activate weapon properties or score critical hits which require a success with +2 or +3 advantages, basically forget about it unless you have a very large dice advantage. The odds of a less-experienced character with good skill (a dice pool of 1g+2y) activating her twin-linked cannons on a typical shot (difficulty 2p) are only 9%. The odds of a crit are only 2%. A ridiculously skilled gunner on that same shot (5y) still only gets +2 advantages half the time, and a crit 30% of the time (remember a crit is generally not “you’re dead”, or even double damage, but something like reducing the target’s speed by 1 or removing a defense die). Also: reducing your crit rating is a really big deal. A vibrosword with a mono-molecular, serrated edge is extremely nasty (vicious 2, crit 1). Very nearly as good as a lightsaber.
• Quantity of dice is better than quality of dice. In virtually every case, your odds of success with N green dice are better than with N-1 yellow dice. For Piloting checks, someone with Agility 3 and skill has worse odds than someone with Agility 5 and no training until the former has trained up to 5 ranks in Piloting. There are minor exceptions when a skill level is high and characteristic ratings are only different by 1, but they are quite small. A boost die has a bigger impact than a die upgrade, and it can significantly increase your chances of success and critical success.
• In the same vein, upgrading a purple difficulty die to a red challenge dice just isn’t that big a deal. Outside of the triumph and despair symbols (see next), the effects of the yellow and red dice on your chances of success vs failure or advantage vs threat are negligible (they help; just not a lot). In most situations a challenge die is about a 5 percentage point hit on your chances of success, with minimal impact on your chances of critical successes (again, outside of despair, which I’m getting to). Adding a setback die is significantly more impactful than upgrading a check, in that it has a somewhat greater negative impact on your chances of success, and a much nastier hit to your chances of critical success (roughly halving them in the 1g+2y vs. 2p case).
• By far the most important impact of the proficiency (yellow) and challenge (red) dice are the 1:12 chance for a triumph or despair.  The 1g+2y shot vs. 2p has only a 3% chance of a critical, but it has a 12% chance of a success + triumph. For anything that requires three or more advantages to activate,  triumphs are the way to go. Even when you are enormously skilled and the task is easy, you are still more likely to get triumphs than 3 advantages. The overall impact of the challenge dice on the game (and so the despair symbol) is less pronounced simply because there aren’t very many of them flying around. You’ll occasionally get them through the GM spending Destiny, and a Nemesis’ Adversary talent; that’s about it.
• The bias for failure + advantage and success + threat is very noticeable when the pool is balanced (equal green and purple dice). Due to the extra threat and missing failure symbols on the purple dice (compared to the green dice), until you have a big dice imbalance success with threat tends to dominate the nontrivial results, with failure plus advantage coming next, and then failure plus threat and success plus advantage only filling up as the dice become overwhelming in one direction or the other.

Some of this makes sense, some of it is decidedly odd.

I am definitely not a fan of how it makes levels of skill relatively unimportant. You might think 3 ranks of Piloting gives you some niche protection in that area; but it does not, someone with 4 Agility is basically as good as you. High characteristics are a big deal, and spending any of your initial XP on anything other than characteristics doesn’t make a ton of sense, from a pure min-maxing perspective. This is not great from a “making characters interesting” perspective.

The flipside of the relative weakness of skills is the imbalance in the Destiny Pool. For the GM, spending a dark side Destiny Point to upgrade the difficulty of a skill check just doesn’t do a lot – it’s usually about a 5% hit, the equivalent of a -1 in a d20-based system, with a small (8%) chance of a despair symbol. Barely worth the effort. The FATE-like “Luck and Deus Ex Machina” function of the Destiny Pool is great and I like it a lot. But spending points to upgrade checks and difficulty is fiddly and low-impact.

In fact, many of the parameters of the game seem to be built on a profound misapprehension about how powerful the challenge and proficiency dice are and the frequency of surplus advantages. There are weapons with critical ratings of 4 or 5, as if the chances of that level of surplus did not round to zero. In table 7-5, which talks about how to spend advantage and triumph in starship combat, two options for spending triumph are: “Do something vital to turning the tide of the battle, such as destroying a capital ship’s shield generator or losing a pursuing ship in an asteroid field”, and “Upgrade an allied character’s next Piloting Gunnery, Computers, or Mechanics check”. Which is deeply weird. That first use of a triumph is obviously extremely powerful – much more powerful than a simple critical hit. Yet for a character with skill, getting a triumph is much more likely than rolling a critical hit! The second effect, on the other hand, is comparatively trivial. Not only that, but it’s usually weaker than adding a boost die to the same check, which you can do with a single advantage! The proficiency dice do come with triumph symbols which boosts do not, which complicates the comparison slightly, I think the general rule holds. Boosts increase you chances of success more than proficiency dice do.

One of the interesting ways in which the dice pool works that did make sense is how armor affects defense. Armor is generally modeled through setback dice, and the interesting thing as that while it’s generally a modest reduction in your to-hit chances (on the order of 8%), it really nerfs your chances of getting surplus advantages (the impact on triumphs is less noticeable).

At the end of the day I’m not sure what to make of all this. I like the dice pool mechanic quite a bit, and if you look at it as a core system, it has lots of interesting features and it’s fun to roll lots of colorful dice. For purely narrative checks it works great, as long as you have a basic grasp of the frequency of some combinations. However, a lot of the crunchy superstructure built on top of it seems deeply suspect.

In the long term, to make a more smoothly functioning game, I think we need a serious recalibration of many of the parameters of the game system. Fortunately, it’s not too hard to figure out some tweaks to make your game run better. Here is what I’ve taken away from it for my GMing:

1. Be very generous in handing out setback and boost dice; they are the most interesting dice to add to routine checks. Use the boosts like candy; give them to players as a reward for trying something cool cinematic, in addition to their modeling function. They are fairly strong, and in order to get surplus advantage to trigger interesting game effects (primarily in combat), characters are going to need the extra punch provided by boosts. The point of diminishing returns on the chance of success is at about 85-90%, so dice beyond that point (roughly +3 dice, although obviously the system has a lot of variability) are mainly going into generating advantage. A lot of talents also remove setback dice, so in order for that to be interesting you need to be handing them out fairly routinely. As a corollary for players, all those things you can do to gain boost dice in combat (aim, spending advantage) are worth doing.
2. Don’t overdo the triumph symbols, either narratively or in combat. When characters are skilled, triumphs are not that uncommon (12-15% for a balanced dice pool with a couple Proficiency dice). Feel free to go nuts with multiple triumphs, but a single triumph should not be allowed to dismantle a scene or conflict.
3. Be aware of the system’s significant bias for success with threat and failure with advantage in routine checks. This is actually mostly a feature, not a bug, and allows you to make success more complicated and (more usefully) mitigate failure. But it does get repetitive, so don’t get too worked up about it; it’s OK for the benefits of rolling an advantage or two on a failed check to be small and transitory, since it’s very common. It also argues for rolling dice only when it’s genuinely interesting to do so, but that’s good advice for any game under any circumstances.
4. The Destiny pool doesn’t really work, because the impact of a single challenge or proficiency die just isn’t big enough. I don’t really have a solution for this. The house rule I’m considering using is to have the GM spend them for Numenera-like intrusions as the flip side of the players’ “Luck and Deus Ex Machina”, but what the system seems to really want is just a much more potent die.
5. Find ways to get more challenge dice into the game, just so you can play with despair. As written, the system favors adding setback dice to modify difficulty, which I think generally makes sense, but we’re just not rolling enough red dice and they don’t have enough impact on the game. While excess threat is generally easier to find then excess advantage, we still want to see those despair symbols occasionally! Consider giving more opponents the Adversary talent. Minion groups especially currently really suck, and giving more imposing ones (Stormtroopers, TIE wings) some kind of levels of Adversary would help mix things up.
6. Speaking of Minion groups, they do really suck because the extra proficiency dice they get for being in larger groups just aren’t hugely significant. I think this is fine and generally the intent of the game, but just bear it in mind. Big groups of minions are far more imposing on the page than they are in actual play. The number of groups is far more important than the numbers in each group.
7. During character generation, I like to give players an extra 40XP to spend after spending their initial allotment (I think this number could actually be even larger). The system is so heavily biased in favor of characteristics that players are going to sensibly spend as many of of their initial points as possible on those. Making sure your character has one characteristic of 4 is a huge deal. Low characteristics, 1 or 2, can limit you because they cap the number of yellow dice you can roll, and so in the long term limit your ability to generate triumphs no matter how skilled you become. Giving players some points they must  spend on skills & talents to differentiate the characters seems wise. In an unrelated point, characters need a lot more starting money – 1500 credits instead of 500 seems closer to right. Yes, there are options for spending Obligation or Duty for more equipment, but messing with this is awkward and 500 credits is just ludicrously low.

This may all sound negative, and I do think it’s true that there is too much in the game that just doesn’t make much sense as designed. Unfortunately, there are a lot of things in the game – weapon ratings, talents, advantage spends – that clearly are at least somewhat misaligned. But, the cool thing about the system is that fundamentally it’s quite simple and elegant and has a lot of potential. Once you’ve gotten a handle on how things actually work, it’s not that hard to hammer things into some form of order. I still need to find a fix for the destiny pool, but I feel like as I play more and get more of an understanding of how the pieces fit together, and how to use them properly (even if that’s not exactly what the rules say), I’m happier with the game. While it’s always going to be a game in need of constant tinkering to keep working, that tinkering is not particularly onerous, Star Wars is fun, dice are fun, and the core system is good.