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A “not too horrible” general gaming podcast in production for more years than we care to admit.

Dice are our vice.

Probability & The D6G Rating System

Sep 11, 2009

Special thanks to listener Bill Roberston who sent in the following details about our rating system and how the re-rolls really stack up.


No, I'm not a math nerd.  I was forced to take statistics in school,
and that included a healthy dose of probability.  The first part of
most probability courses covers the binomial distribution, which tells
you if you try something a bunch of times, what are the odds that 1
try will succeed, or 2 will succeed etc...

My little gamer mind immediately latched on to this, and only this,
part of the course!

For example, if I want to know the chance of a 3+ with a reroll.  I
first start with a google search for binomial calculator.  Next, plug
in 2 tries with a .67 success chance.  Calculate the result and ask it
to tell me the chance of getting 1 or more successes.

So out of curiosity, I used the web calculator to figure out all of
the d6 with a reroll values and ranked them together with the
single-roll odds.  This is the one I used.

2++ = 97%
3++ = 89%
2+  = 83%
4++ = 75%
3+  = 67%
5++ = 56%
4+  = 50%
5+  = 34%
6++ = 31%
6+  = 17%

+ = w/o a reroll
++ = with a reroll

I don't use it very much, but its very handy when coming up with house
rules to see what the effect of multiple rolls would be.  Or, if I
feel like a baby, I use it to see how fast animosity will screw my Orc
army as it gets larger.