# [MUD-Dev] Chances of success (was d20 system)

Bobby Martin bobbymartin at hotmail.com
Wed Aug 15 21:42:50 New Zealand Standard Time 2001

```> From: "Brian 'Psychochild' Green" <brian at psychochild.org>

> Why cripple yourself with that sort of low granularity?  Why not
> increase the granularity to something that people rolling dice
> would find unwieldy?  For example, use percentages.  Come up with
> reasonalbe rules for automatic misses and critical hits.
> Something that isn't limited to steps of 5% granularity.

> For example, in Meridian 50 we used a "d1000".  We had several
> factors that weren't quite so easy to calculate, but that were
> still intuitive to the player (IE, having higher weapon skill
> allowed you to hit more often.)  Using a d20 based system would
> have made the combat system worse, IMNSHO.

In fact, why go with a dX system at all?  If you have some
familiarity with math, you can use a system that gives you the
behavior you want, and drop worrying about how it maps to an N-sided
polyhedron.

For example, in Cosm we decided we want the chance of success on a
skill to be proportional to the ratio (skill level)/(skill level +
difficulty).  This way an increasing skill increases the chance of
success, but there is never either a certainty of success or
certainty of failure.  Neither is there a minimum chance of success
or failure; it just in/decreases asymptotically as your skill
in/decreases relative to the difficulty.

We also decided that the strength of success/failure should be
essentially unbounded, so that 90% of all '1' effectiveness
successes will be upgraded to 2, 90% of all 2 successes upgraded to
3, etc.  In fact I use logarithms to get all of this from a single
random number in the range [0,1], which is about as close to a die
roll as this system gets.

Anyway, the gist of what I'm trying to say is, rather than pick an
arbitrary system and tweak it, why not decide the characteristics
you want your success system to have and produce an algorithm with
those characteristics.  You may still need to tweak, but then it's
because you misunderstood what you wanted rather than because of
systems tend to avoid the scalability problems you see e.g. in AD&D
at high levels (a d20 doesn't have nearly enough sides : )

Remember, you only have to understand how to code the system, not
actually calculate it every time a success is tested in your game.

I can give the details of our success system if there is interest.

Regards,
Bobby Martin
Cosm Development Team

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