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Probability calculation

Started by wasd May 20, 2009 at 5:54 AM 1 reply 900+ views

Original Post

May 20, 2009 at 5:54 AM

I'm trying to calculate the amount of things that die from natural causes. At the moment, I've got this: For each thing, I do: if (Random number between 0 and 1) <= (Death probability) (Number to kill off) += 1 Which works fine and is exactly what I want, but I was wondering if there was a way to get the same kind of results with just a single calculation, e.g.: (Number to kill off) = (Random number) * (Death probability) * (Total number) The problem with the above is that it produces a lot lower numbers, and when there are a few things (between 1 and 100 say), you don't get any being killed off.

SiCrane

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SiCrane

May 20, 2009 at 6:12 AM

The sum of discrete random variables can be approximated by a gaussian distribution with the same mean and variance. Assuming each entity has the same probability of dying P with a total number T, then the mean would be P * T and the variance would be (1 - P) * P * T. Plug those values into your favorite gaussian random number generator.

alvaro

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alvaro

May 20, 2009 at 9:50 AM

What you are asking about is called binomial distribution.

A normal distribution is often a good approximation to the binomial distribution if you have enough events. If the probability of individual events is very small, a Poisson distribution is a better approximation.

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