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I'm a professional game developer who has worked on a number of large titles but went indie a few years ago. I'm about to release my 3rd game, a real-time strategy game with a few RPG elements.

My problem is this: what is the best way to express the result of a percentage chance to a player? What I mean is, if the user has a 15% chance of succeeding at something, and the game engine uses a random number generator to generate a percentage, how would the result be shown to the user so that it is easy to understand?

The challenge I have is that if the player has a 15% of achieving something, if the random number generator generates a number between 1-15 (say, 4), then the player succeeds. But expressing this as, say, 4% / 15% just doesn't look right as conversely a lower number means success, where most players are used to "higher numbers = better".

I have thought about inverting the equation and expressing it some way as a chance of failure. So in the above example the player would have an 85% chance to fail, where the 4% would be converted to 96% and the result could be expressed as 96% / 85%, which "looks" more correct. But I just don't like the idea of expressing everything as a chance of failure, rather than a chance of success.

I have also thought of simply expressing the success chance (85%) and not showing the player the result of the random number generator, but I feel that most players would want to see the numerical result rather than simply the victory/failure condition.

I'm at a real loss here and would love to hear the views of players and other game developers alike. Even better if someone can point me to an example of a game which has solved this problem!

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    \$\begingroup\$ Is representing the roll in numerals a requirement, or would you be interested in exploring graphical treatments like a slider/spinner that stops in/outside a target band? \$\endgroup\$
    – DMGregory
    Commented Oct 17, 2017 at 13:09
  • \$\begingroup\$ A graphical treatment is a great idea, but we are very close to a closed alpha so a textual output would be much faster right now. \$\endgroup\$
    – Arj
    Commented Oct 17, 2017 at 22:40

4 Answers 4

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You can strip extra information and display everything in uniform fashion.

How about sci-fi d100 dice for everything?

You need to roll more than 85. You have rolled .
You need to roll more than 85. You have rolled ..
You need to roll more than 85. You have rolled 4. Bad luck ..

You need to roll more than 85. You have rolled .
You need to roll more than 85. You have rolled ..
You need to roll more than 85. You have rolled 86. Congratulations!

  • This way player can quickly get used to seeing numbers he needs to roll.
  • The dice reinforces the feeling of randomness of the result.

You don't say much about visual part of the problem - where in UI this is, how much space does it use, etc., so mockup can not be easily made.


Just in case - d100 dices do exist:

enter image description here

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  • \$\begingroup\$ Thanks for your answer. I am leaning towards this type of implementation as I want to keep the approach text-based for now. It's not as short as a 4% / 85% type solution, but it is much easier to understand. \$\endgroup\$
    – Arj
    Commented Oct 20, 2017 at 2:29
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This can be expressed in terms of the Dungeons and Dragons mechanic, where the player must roll above a specified target number on a 20-sided die to succeed in a task. Various modifiers can be applied to the roll to simulate factors such as ease or difficulty, skill levels, environmental factors, etc.

For the specific example of 15% chance of success, this can be simulated by needing to roll above 20 but with a +3 modifier.

Advantages of this scheme include:

  • Higher numbers are always better.
  • A positive modifier increases the chance of a positive result.
  • The degree to which 20 is exceeded can be used to simulate a degree of success.
  • It's a known, proven system that players may even be familiar with.
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  • \$\begingroup\$ Hi there and thanks for your answer. I'm very familiar with the d20 mechanic having been a D&D player for many years. I'm consciously trying to steer away from it for a few reasons: 1) The d20 system is used more in fantasy RPGs - this one is more sci-fi where percentages give more of that techie "look and feel" 2) I need the system to be more granular than 5%, with 1% being about right. Hence the percentage system 3) While I agree that many players know about d20, literally everyone knows how percentages work \$\endgroup\$
    – Arj
    Commented Oct 17, 2017 at 12:32
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Whilst this may only affect some uses, perhaps it would be possible to indicate the randomly generated number as a "resistance score".

To use your example:

  1. The player has a 15% chance of success when performing some action on some target.
  2. The target resists, because inertia, evasion, difficulty, etc.
  3. You display this resistance attempt to the player.
  4. The player's action succeeds when the target fails to resist.
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  • \$\begingroup\$ You're right, this really only works for "opposed" checks so I can't apply it as a universal/core mechanic. Also, my question is more about how to visually display the result against the possibility to the player, rather than the workings of the mechanic itself. \$\endgroup\$
    – Arj
    Commented Oct 17, 2017 at 22:38
  • \$\begingroup\$ This can be displayed in the same way for either, as you put it, opposed checks or one-sided checks. Imagine having a gauge filling from left to right, with a target bar that represents success chance, and fills based on the "resistance" to success. (Look at Xcom 2's hacking percentage for an example of how this looks: it feels intuitive as a player, and shows all the information you were wanting to display.) A textual representation would work the same way: the random number incrementing from 0 to whatever value it is given, with the target number beside it. (Sabotage Attempt: x% / 85%) \$\endgroup\$ Commented Oct 18, 2017 at 8:06
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Thanks for the replies. It seems there is no simple, straightforward way to present this information. I could possibly solve it by being more verbose in-game, but this is very awkward. Using my 4% / 15% example above it would look like:

You have a 15% chance of success, so you need to score 85 or above to succeed. You score 96 - success!

This makes sense to the new player but is way beyond a simple numerical display and I'm sure would get annoying for experienced players.

My solution, unfortunately, is to simply display the probability (e.g. 15%) and only show the player the outcome (success or failure) rather than the numerical result. My inspiration for this is Fallout 3, which shows the % chance of hitting a particular body part of a foe, but then only graphically displays the result of the action - it never gives you a numerical result. Sometimes, maybe, less is more :)

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  • \$\begingroup\$ Is there really a reason to display the result? What benefit does the user gain from knowing exactly what random number won/lost them the roll? \$\endgroup\$ Commented Oct 19, 2017 at 17:34
  • \$\begingroup\$ In most games players like to see the result of a random number, especially if it is against a specific number they were trying to attain. D&D and all its computer-based variants are the model for this. As I mentioned some games (e.g. Fallout) don't show the numerical result, but I think this is more uncommon. \$\endgroup\$
    – Arj
    Commented Oct 20, 2017 at 2:31

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