The Aurora RPG Engine – Part 4

Fixed Target Number and Difficulty as Dice Modifiers

There is one main disadvantage to most dice pool systems:  the time it takes to total the rolled dice.  This begins to get especially tedious around 7-8 dice and only increases thereafter, slowing the game and potentially killing its momentum.

A second disadvantage also arises if the system uses a series of increasing target numbers (for example, setting a target number of 10 for an Easy task, 15 for a Moderate task, 20 for Difficult task, etc).  Doing so undermines the intuitive feel of “Number of Dice = Chance of Success”.  If I have 8 dice in my hand versus 5 dice, I should feel as though my character is more capable; however, if the target number is also changing/increasing, then it’s tough to gauge whether those extra dice really are leading to an extra chance of success.  With two variables at play it creates a matrix of possibilities that hinders any automatic and visceral feel.

By setting the target number at a fixed value of 15 and by adjusting both for the difficulty as well as accounting for all other modifiers by adjusting the number of dice rolled:

  • The number of dice rolled at one time is generally kept low.
  • Even when many dice are rolled, we only need to count enough dice to make 15, which can be easily done by adding together the highest dice until 15 is reached. (The remaining dice are used as a “Margin of Success”, to further explained below.)  This avoids needing to tally large numbers of dice.
  • And because the target number never changes, we easily grasp the chance of success by the number of dice in our hand. The visceral nature of the die pool is maintained.

Why Not Use A ‘Success’ Mechanic Instead of Counting at All?  One of the common ways to avoid the “count many dice” disadvantage of a dice pool is to use the concept of “success dice.”  These are either custom dice where a certain number of faces are explicitly labelled as successes, or by using regular dice and denoting which values count as a success (for example, stating that every 5 or 6 on a die counts as a success).  In this way, instead of summing the value rolled from every die we are only looking for successes and counting those.  As a bonus, since we’re already dealing with successes this also allows an easy way to incorporate a Margin of Success mechanism.

That sounds great and easy, so why not do that?

In short, it is because of the “law” of averages.  This explanation might get a bit involved, so please bear with me… but in short, using a success die system reduces number of combinations on the dice rolled that potentially can lead to success.  This is because low rolls are not aided or averaged out by high values on other dice.  For example, if we roll 6 dice in a system where 4+ on a die equals success, and we need 3 successes, and get a roll of 6, 5, 1, 1, 1, 1, this would count as 2 successes and thus a failure.  However, under a more traditional additive system with a target number, the two high numbers help carry the low rolls, averaging things out to count for at least a baseline of success (and using Aurora’s target number of 15, it would indeed be a success).

The net result of a using a success dice regime is a reduction in the number of successful die roll combinations, leading to greater jumps and gaps in the chances of success or failure between each die that is added or subtracted from the pool, making for a probability curve that is way steeper than I like or want.  (I did a lot of modelling using dice probability calculating websites…)  And these jumps only become more pronounced as more dice are added to the pool.  While this can be partially overcome with certain tricks (such as saying certain values or faces on a die counts as multiple successes), these tricks are cumbersome and often require players to invest in specialized dice.  These are all downsides when compared to the straightforward “Additive, Count to 15” motif that lies at the heart of Aurora.

Base Underpinnings and Dice Pool
| Index |
Baseline Values and an Aside on Probabilities

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