Calculating Binomial Distribution: Probability of Third Strike on Fifth Well

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Discussion Overview

The discussion revolves around calculating probabilities related to the binomial distribution, specifically focusing on the probability of the third strike occurring on the fifth well when drilling for oil, given a strike probability of 0.2. Participants explore various approaches to this problem, including expected values and combinations.

Discussion Character

  • Mathematical reasoning
  • Exploratory
  • Technical explanation

Main Points Raised

  • Brendan introduces the problem of finding the probability of striking oil three times out of five wells, using a binomial coefficient and probability calculations.
  • Another participant suggests that the probability of the third strike occurring on the fifth well can be calculated as the product of the probability of two strikes in the first four wells and the probability of a strike on the fifth well.
  • Brendan calculates this probability using combinations and the given strike probability, arriving at a value of 0.03072.
  • Brendan questions the expected value of striking oil three times in succession, proposing a calculation of 0.008 based on multiplying the strike probability three times.
  • Another participant clarifies that the expected value calculation is for three successive strikes out of three tries, suggesting that the probability increases with more tries.
  • Brendan later revises his approach, indicating that he should use the negative binomial distribution to determine how many wells need to be drilled to achieve three successive strikes, proposing a formula involving the number of successes and the probability of success.

Areas of Agreement / Disagreement

Participants express various methods and calculations regarding the probability of strikes, but there is no consensus on the correct approach or final probability values. The discussion remains unresolved with multiple competing views on how to calculate the probabilities.

Contextual Notes

Some calculations presented depend on the interpretation of the problem, and assumptions about the independence of strikes may not be fully articulated. The use of different probability distributions (binomial vs. negative binomial) introduces additional complexity that is not fully resolved.

brendan
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Hi Guys,
I have been given the probability that a drill strikes oil in a region = 0.2.

I know that if I wanted to find the probability of say striking oil 3 times out of 5 wells

It would be 5Choose3 = 5!/((2!)(3!)) * (1/5)3* (4/5)2 = 0.0512


My question is how would I go about finding the probability that the third strike occurs on the fifth well?

regards
Brendan
 
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That would be the probability that two strikes were found in four wells times the probability that the fifth well is a strike.
 
So,
It would be 4Choose2 * Px=0.2 = 4!/((2!)(2!)) * (1/5)2* (4/5)2 *(1/5) = 0.03072
regards
Brendan
 
Is the Expected value of striking oil 3 successive times just 0.2 * 0.2 * 0.2

= 0.008 ?
regards
Brendan
 
brendan said:
Is the Expected value of striking oil 3 successive times just 0.2 * 0.2 * 0.2

= 0.008 ?
regards
Brendan

That's the probability of striking oil three successive times out of three tries. If you have five tries, the chance is higher:
SSSnn
SSSnS
SSSSn
SSSSS
nSSSn
nSSSS
nnSSS

where n is nothing and S is a strike. Your probability covers the first four cases, but the following three cases are also possible.
 
So,
Let say I have 5 tries there are 5P3 = 20 ways of getting 3 in a row.
and there are 5! = 120 combinations. so would that make the probability 5P3 divide 5! = 1/6 ?

regards
Brendan
 
Sorry guys,
I should have said how many wells would have to be drilled to strike oil 3 times in in succession.

I should be using the Negative binomial for that calculation.

It is the number of successes k divided by the probability of success p

so k/p = 3/0.2 = 15 wells drilled before success
 

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