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Generating Random Numbers with Specified Probabilities

Date: 08/10/2003 at 16:54:06
From: Xeon
Subject: Generating Random Numbers with Specified Probabilities

Given an evenly distributed Pseudo Random Number (PRN) generator (with 
each generated outcome having the same probability), how can I 
generate outcomes with specified probabilities?

For example, I want to generate these outcomes:
   A with 20% probability
   B with 15% probability
   C with 30% probability
   D with 15% probability
   E with 10% probability
   F with 10% probability

As far as I know, the simplest approach is to use the PRN generated by 
the evenly PRN generator to index a single dimensional array with 
elements matched to the specifications. For the example above, this 
would mean the array would have 20 elements with the value of A, 15 
elements with the value of B, 30 elements with the value of C, etc. 
The PRN generator should then be set to generate random values in the 
range of 1 to 100.

The above approach is out of the question for my current needs. Are 
there any other ways to achieve the same effect?


Date: 08/12/2003 at 04:11:32
From: Doctor Jacques
Subject: Re: Generating Random Numbers with Specified Probabilities

Hi Xeon,

If you have only a "small" number of cases, as in your example, you 
should make a table of cumulative frequencies. For example, the 
probability of A should be 0.20, the probability of (A or B) should 
be 0.20 + 0.15 = 0.35, and so on. The nth row in the table gives the 
required probability of selecting an element less than or equal to 
the nth element (assuming a standard ordering of the elements).

In your case, this gives the table:

  A  0.20
  B  0.35
  C  0.65
  D  0.80
  E  0.90
  F  1.00

For example, the row C tells us that we should select (A or B or C) 
with probability 0.65.

You can then use your PRNG to generate a random variable x between 0 
and 1, and look in the table to find in which slot it falls, i.e. you 
find the first table row with a value greater than x. For example, if 
you produce x = 0.47, the first row greater than 0.47 is C (0.65), 
and so you will output C. If the table is somewhat large, you can 
find the correct row rather efficiently by using the binary search 
method (write back if you want to know more about this).

Does this help?  Write back if you'd like to talk about this 
some more, or if you have any other questions.

- Doctor Jacques, The Math Forum
  http://mathforum.org/dr.math/ 
Associated Topics:
High School Calculators, Computers
High School Probability

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