Wednesday, April 21, 2010

Probable Probabilities

I'm a sucker for dice and other mechanics. I see (in games, on blogs) many interesting mechanics and think up my own crazy schemes from time to time.  I often wonder about the probabilities behind these mechanics.  I've done some "analysis" in the past.  I'm doing more.  All will be indexed in this post.

["analysis" is in quotes cause 1) I'm no statistician 2) I'm gonna mostly use brute force programs (i.e roll 1,000,000 times, avg result) rather than math which is more exact but math is hard&work where as programming is easy&fun.]

Probable Probabilities
  • Ye old bell curve.
  • Exploding dice.
  • Hackmaster's correct exploding dice.
  • AC Bonus vs Save.
  • 2d6+mod > value.
  • 3d6+mod > value.
  • Roll X dice take highest.
  • Roll X dice take lowest.
  • Party rolls vs individual 2in6, etc. see Party Rolls.
  • 2d6 Thief'n from RPG Characters.
  • Forget where this came from Instead of using a percentile die for Thieves' Find Traps etc., you could give them more dice, depending on their level. Start them out with 2d6, and add a d6 every 3rd (?) level. For consistency, I'd do this with Elves and Dwarves as well, so rather than finding a trap on a 1-2, they'd get two dice to roll and find the trap if either of the dice showed a 1. Incredibly cunning traps might require 2 ones showing to be revealed, or even more for truly diabolically sneaky traps.
  • To be continued...  (requests welcomed)


  1. <- this might prove incredibly usefull to you.
    Not only can it roll dice, it also calculates probabilities really well.
    The calculator can handle exploding dice, as well as a large ammount of other stuff, like: Roll 5d6 pick best 3 dice.
    Maybe you already know the Troll, but some of your readers might not...

  2. Well that takes some of the fun out of my plan...

    Cool tool.

  3. The lone d6 rolled behind the screen is a convenient and rapid way for the GM to determine success or failure at a task not specifically outlined in the rules. This is accomplished by mentally accessing the LIKELYHOOD of success, then if the d6 result falls in the range listed (lower is better), the character achieves the desired goal.

    Re roll all ‘1s’ if character is a novice (1st level or less)
    Re roll all ‘6s’ if character is accomplished adventurer (6th level or higher)

    ALWAYS (6 or less)
    USUALLY (5 or less)
    OFTEN (4 or less)
    FREQUENTLY (3 or less)
    SOMETIMES (2 or less)
    UNLIKELY (1 or less)
    RARELY (0 or less)
    PREPOSTEROUS (-1 or less)
    IMPOSSIBLE (-2 or less)

    Typical Modifiers

    +1…..Player utters these words, “I am an elf”

    -1……Player gives detailed explanation, or

    -2……Player’s explanation makes GM laugh

  4. Collect a few of these and I'll promote this to my favourite blog. ;)

    I always ask my wife about some of those, and I have a very hard time to get the ideas into my head. I'd love to be able to understand probabilities.


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