Statistics question

The Vigilante · Post by **The Vigilante** » Wed Sep 09, 2009 3:39 am

OK, so my statistics are a little rusty, and my textbooks arent't handy at the moment and google doesn't seem to like me either. I'm sure this'll be easy for you guys but I'm stumped right now. I'm trying to use Excel to calculate the outcomes of hitting a certain number or better with a d6, compared against a target number of success.

Where I am now :
I know that the probability of, say, getting a 5 or a 6 on a d6 is 2/6.
Getting at least one 5 or 6 with a certain number of times is 1-(4/6^n)

What I want to do :
Calculate the probability of having a certain number of 5's and 6's with a certain number of dice.

For example, I want to have at least two hits, with a dicepool of 6 dice.

How do I go along and calculate the probabilities of this happening ?

Heath Robinson · Post by **Heath Robinson** » Wed Sep 09, 2009 4:05 am

That's a simple binomial distribution question. You have N tests (i.e. N dice) and want to calculate whether you get R successes (i.e. R hits) then the formula is nCr * p^r * p'^(n - r). P is the probability of success (i.e. change of getting 5/6) and P' is the inverse of that probability (i.e. 1-P, the chance of not getting 5/6).

Excel has a function called BINOMDIST to save your fingers and brain. I believe it defaults to the cumulative form, so you should probably set the final parameter. Use the function wizard; I can't remember the parameter order.

Doom · Post by **Doom** » Wed Sep 09, 2009 4:07 am

Try looking up 'binomial distribution', pretty sure Wiki has a nice page on it.

You're basically asking for n trials, with a 1/3 chance of success on each individual trial, and counting successes, which is easily addressed via a binomial distribution calculation (and there might be charts somewhere to save you some calculator button pressing).

My TI-83 has buttons for it, so if you have a decent calculator, it probably has it as well.

shau · Post by **shau** » Wed Sep 09, 2009 5:31 am

Or you can do like us weak math people and just assume the dice will always fuck you when you need them most.

Crissa · Post by **Crissa** » Wed Sep 09, 2009 5:57 am

Shau, that's a truism whether or not you're a math geek.

You should see my spouse roll dice.

-Crissa

The Vigilante · Post by **The Vigilante** » Wed Sep 09, 2009 2:32 pm

shau wrote:Or you can do like us weak math people and just assume the dice will always fuck you when you need them most.

Yeah, that's that I'd do usually, but the point of all this is to create a gaming sysem where the probabilities match what I want to happen.

Thanks for your help everybody, you've been most kind.