Stepped Dice vs Dice Pool

One of the things that I noticed when I first read Coriolis was the table that set out chances of success with different numbers of dice in your pool. According to the table, roll one die and you have 17% chance of success. (Let’s ignore for the moment the ridiculous concept of “partial” success, which appears only in only Coriolis – in my games one 6 is s success. That is all you need. With ten dice in your pool (lets imagine a 5 in your stat, 3 in your skill and a couple of business dice from your gear), the chance of success with your first roll is 84%. Which might not seem much, but according to the table if you push your roll you have 96% chance of success.

Yes, yes I have heard all the stories of people rolling and pushing 12 dice or more and not getting a single six, but that my friend, is a thing called probability.

Obviously these tables of probabiltity, which feature in all the books have limited value. The one in Coriolis for example takes no account of preparatory prayer, which grants an extra die or two on the pushed roll. Indeed, I think the table in Coriolis (at least in my early printing) is actually wrong – if I recall correctly this version of the table assumed that 1s would not be re-rolled, and so the chances of success on a pushed roll are slightly lower than they should be. Indeed turning to a more recent version of the year Zero Engine, Veasen, we can see that, though the probabilities for the first roll are the same, the chances of success on a pushed roll are much improved. In Coriolis the table states you have a 29% chance of a success on a single pushed die. Vaesen says it is 31%. And the chance of success on 10 pushed dice rises a point to 97%.

But recently, in Twilight 2000 and Bladerunner, Free League have moved away from d6 dice pool. Instead a player generally roles two dice – each one between varying between a d6 and a d12. The eight sided dice gives you a success of a six seven or eight. The d10 not only offer successes on everything abouve a six but roll a ten and you get two successes. The d12 offers two successes on 10, 11 and 12. How does all this change the odds? 

When we are talking about basic chances of success, not much changes. Let’s look at the tables. Of course the odds on a single d6 remain the same, but what does 2d12 give you? Well that gives you 82% chance of success on single roll and 97% on a pushed roll. That pushed roll is the same as rolling 10d6, but the unpushed roll gives you a slightly lower probability of success. 

That might sound ‘almost exactly the same but not quite as good as rolling 10d6’ but having played in games of Twilight2000 and Bladerunner, I think I have rolled 2d12 a lot more often (proportionately) than I have rolled 10d6. Think about it – even if you are rolling five dice for your attribute, you rarely roll five dice for skills – yes you might get one or two more for gear, but its a stretch to roll ten dice. 

With the stepped dice system, it is possible to end up rolling just one dice – indeed its a lot more likely that with a d6 pool. Unless you are really wrecked in, say, Forbidden Lands you will be rolling 2d6 at least, because that’s the minimum attribute you can have. In the stepped die system, it is possible to have an attribute of just d6. But because one attribute dice could be a d12, you chances are not that bad – 17% as usual for a d6, but of course a d10 gets you a 50/50 chance, and a d12, 58%. And that’s before you push the roll. Then the chance of success on a single d12 goes up to 82%. 

But there is another effect of the stepped die system. While you can end up rolling just one die – you can’t roll any more than 2d12. In the d6 dice pool version, even though the table only goes up to 10d6, a player can end up rolling more than 10. Not often as I have already said, though certain games, like Alien, make that a more likely possibility. If you modify your roll, for example, by getting help from fellow PCs, you might well add 3d6 to your roll. Add that to, say, five in an attribute, two in a skill, and another two from your tools, and you are rolling 12 dice. In the stepped dice system, let say the equivalent o that attribute is A, or a d12, and your midlevel skill is C, so a d8. Your equipment generally gets you +1 in the stepped die titles, and what that does is boost your d8 one step to a d10. Now your three friends offer to help, each helping PC gives you a +1, but that’s one step, not an extra die. You are already at d12 on one die and now at d10 on the other, so the only help you can take is one step putting the d10 put to a d12. In Twilight 2000 you can add ammo dice to shooting rolls, but other than that, 2d12 is as good as to gets. 

I want to conclude this piece with what I think might be the one real disadvantage of the stepped die system, but before I do that I want to explore what the slightly reduced range of possibility (rather than probability) means for character skills. Around our table, I have not noticed very much skill inflation, people spend most of their XP on talents. But the most dice you can have in a skill is five. And the fewest is none at all. Overall there they’re a six ranks of skill. In the dice pool games there are only five. The four dice steps and of course none. The increased chance of success going up one rank from zero to one is the same in both systems. But what is the difference in going from 1 to 2?

Ignoring your attribute for a moment, and pushing, the two dice in the old system give you a 31% chance of success, but a d8 in the stepped system gives 38%. If you are a player, you might be going “woo hoo” but be warned – if you get your skill all the way up to grade A or d12, that only offers you 58% chance of success, while five dice get you 60% in the old system. On the other hand you do spend more XP getting there. On the other other hand (three hands? Are we playing Mutant Year Zero?) you do have that problem of bonuses from equipment and help being stymied, the more competent you are.

In Bladerunner you do also have an advantage/disadvantage mechanic, which I guess you could apply to a situation like two more PCs than you need offered help:  but that opens up another can of worms.

That is about the consequences of pushed rolls.  It doesn’t apply to every game, but a number have consequences based upon rolling ones, including the two stepped die games. If you are are only rolling two dice, then your chances of rolling a one, are much reduced. Given that rolling a one is generally a BAD THING, this might be a feature players like. But for game designers it takes away the potential for using those ones in interesting ways. Variety that makes a horror game like Alien feel very different from the gritty, detailed combat of Forbidden Lands. It steals a tool from the designer’s kit. And that’s the reason I don’t like the stepped die system as much as the original dice pool.

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