A standard geometric distribution can be interpreted as the number of Bernoulli trials required to get one success. However, what if the probability of success if each trial diminishes by some factor with each failure?

Alexandra Richardson 2022-07-22 Answered
A Diminishing Geometric Distribution
A standard geometric distribution can be interpreted as the number of Bernoulli trials required to get one success. However, what if the probability of success if each trial diminishes by some factor with each failure?
Let p be probability of success of the first trial and d be the diminishing factor of each failure such that the probability of success of trial n is p d n 1 . I have been trying to calculate the expected value of this distribution without success. My main stumbling block is the probability of failing n times:
P ( n  failures ) = k = 1 n ( 1 p d k 1 )
Given that, the expected value would be
E ( X ) = n = 1 n p d n 1 F ( n 1 )
Is there a way to get a closed form for this?
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Answers (1)

Arthur Gillespie
Answered 2022-07-23 Author has 10 answers
Step 1
The random variable X is not well-defined. For any d < 1, there is a non-zero probability that there will never be a success.
To show this, first note that it is clear that for any n, there is a non-zero probability that the game will last at least n trials. We can choose n large enough that p d n 1 is as small as we wish. Given that there have been n trials without a success, the probability that there will never be a success is k = n + 1 ( 1 p d k 1 ). For small p d k 1 , we can lower bound the logarithm of this. For example, it is > n + 1 2 p d k 1

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