The Aurora RPG Engine – Part 5

Baseline Values and an Aside on Probabilities

The base number of dice for a skilled entry-level professional is baselined at 5 dice.  With 5d and a target number of 15, the chance of success is roughly 75% (in actuality it is a bit over, but close enough).  As outlined in this previous blog post, this falls into the probability range that I find is most perfect, where a competent, trained, healthy individual beginning their professional career should succeed on almost-routine tasks (I say almost, as if it were perfectly routine there’d be no need to roll a test!) most times.  And even in those instances where they fail their roll, rarely will it be too disastrous of a setback (especially if the player chooses to succeed at cost, which is further explained below).

By adjusting the dice pool up and down in increments of ½ dice (a d3) allows for roughly this progression of success chances:

3d 10%
3½d 25%
4d 50% (actual is 45%)
4½d 66%
5d 75%
5½d 90%
6d 95%

In graph form, it looks like this (courtesy of

In play, it is easy to remember that 3½d to 5d forms the middle ground of success probabilities.  Anything less than 3½d is super unlikely to succeed, and anything over 5d is pretty much guaranteed to succeed.

Taken together, everything comes together nicely:

  • A nice round target number that is easy to remember.
  • An easily graspable number of dice that counts as “competent”.
  • Enough dice to let beginning characters succeed despite modifiers while also tempting the players with extra actions.
  • And an intuitively graspable notion of the chances of success.

What the Number of Dice Mean in Play.   I have 3 dice in my hand?  My chances are not so great – is there anything I can do to give myself an advantage?  Maybe it is best for me to not perform this action quite yet and instead spend a round setting myself up… or maybe I’ll go ahead anyway, because I’m willing to succeed at cost if necessary! (Thus we have set up interesting choices.)

I have 7 dice in my hand?  Yeah, that’s darn good!  I’ll succeed with style.  I’m going to go for it and use that extra skill to gain some benefits from the high Margin of Success.  Or, maybe I’ll try to do a bit more this round, pulling off a couple of more actions and dropping my dice down to 5d for each action…  (Again, we have set up interesting choices.)

I have 5 dice in my hand?  I’m likely to succeed.  Cool.  But what if I could do better?  Can I get creative to gain slightly more advantage?  Or how about the opposite?  Even at the base of 5 dice, I could be tempted to eek out one more action.  Or perhaps the GM has challenged us with obstacles and countdown events that require me to take multiple actions in a round…  (And so, we have again set up even more interesting choices!)

Impossible Rolls.  What happens when the task difficulty or various modifiers have pushed my pool down to just 2d?  I can do math, there’s no way I can get 15 on two dice.  So why bother, it’s pointless.  Well, not entirely… it’s more of an invitation to get creative!  What can I do in order to improve my chances?  Maybe I can go and do research to learn a bunch of information that gives me an advantage, or find some gear to assist me, or recruit a few allies, or other similar ideas that can and will increase my die pool.  And then there is always the choice to succeed at cost (described below), gaining what I want though hardship.

In addition, in games that include a narrative meta-currency I can always spend my way to success if I really want it.  And in games using a critical-success mechanic, maybe I want to push my luck and hope I get lucky…

Alternately, to come at this from a different direction, maybe it really is out of my league right now.  I can use this as a signal to find a different path, or to come back to it later once I’ve gained more experience.

While a GM is advised to not place too many impossible obstacles in a character (and player’s) way, depending on the campaign style and tone it can be totally appropriate and even lead to crowning moments of awesome.

Granularity Option 1.  If each ‘step’ in the dice ladder is a ½ die, then the probabilities of success between each step can sometimes jump rather substantially.  Between 3d and 6d, the chance of success increases or decreases between 12% per step to 22% per step.  While this, at a gut level, is a tad higher than my absolute preference, I think it remains quite workable and keeps dice accounting simple.

However, for an even smoother shift up or down the ladder, we can insert another step of “die+1” in between the steps of “die” and “die and a half.”  For example:

  • 4d
  • 4d+1
  • 4½d
  • 5d
  • 5d+1
  • Etc…

Which leads to this probability chart (again courtesy of

The downside here is that the dice accounting is not nearly as elegant and requires a bit more care during play to properly adjust the dice pool.  That said, it is still not an excessive amount of complexity and for certain genres and playstyles the extra granularity could be appropriate.

Granularity Option 2.  The opposite side of the coin from Option 1 above, things can be instead simplified to only deal in full dice.  This renders the jumps in probability between each number of dice even larger, thus making each change in the dice pool (advantages, setbacks, modifiers) that much more significant.  While this might push players to become more timid and cautious, it may also push them to be more bold, clever, and daring.

It has the definite bonus, however, of making the accounting absolutely straightforward and ups the visceral feel.  Again, for certain genres and playstyles this could be totally appropriate and welcomed.

Fixed Target Number and Difficulty as Dice Modifiers
| Index |
Handling Difficulty and Modifiers

2 thoughts on “The Aurora RPG Engine – Part 5

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