Answer:
a) There is a 9% probability that a drought lasts exactly 3 intervals.
There is an 85.5% probability that a drought lasts at most 3 intervals.
b)There is a 14.5% probability that the length of a drought exceeds its mean value by at least one standard deviation
Step-by-step explanation:
The geometric distribution is the number of failures expected before you get a success in a series of Bernoulli trials.
It has the following probability density formula:
![f(x) = (1-p)^{x}p](https://tex.z-dn.net/?f=f%28x%29%20%3D%20%281-p%29%5E%7Bx%7Dp)
In which p is the probability of a success.
The mean of the geometric distribution is given by the following formula:
![\mu = \frac{1-p}{p}](https://tex.z-dn.net/?f=%5Cmu%20%3D%20%5Cfrac%7B1-p%7D%7Bp%7D)
The standard deviation of the geometric distribution is given by the following formula:
![\sigma = \sqrt{\frac{1-p}{p^{2}}](https://tex.z-dn.net/?f=%5Csigma%20%3D%20%5Csqrt%7B%5Cfrac%7B1-p%7D%7Bp%5E%7B2%7D%7D)
In this problem, we have that:
![p = 0.383](https://tex.z-dn.net/?f=p%20%3D%200.383)
So
![\mu = \frac{1-p}{p} = \frac{1-0.383}{0.383} = 1.61](https://tex.z-dn.net/?f=%5Cmu%20%3D%20%5Cfrac%7B1-p%7D%7Bp%7D%20%3D%20%5Cfrac%7B1-0.383%7D%7B0.383%7D%20%3D%201.61)
![\sigma = \sqrt{\frac{1-p}{p^{2}}} = \sqrt{\frac{1-0.383}{(0.383)^{2}}} = 2.05](https://tex.z-dn.net/?f=%5Csigma%20%3D%20%5Csqrt%7B%5Cfrac%7B1-p%7D%7Bp%5E%7B2%7D%7D%7D%20%3D%20%5Csqrt%7B%5Cfrac%7B1-0.383%7D%7B%280.383%29%5E%7B2%7D%7D%7D%20%3D%202.05)
(a) What is the probability that a drought lasts exactly 3 intervals?
This is ![f(3)](https://tex.z-dn.net/?f=f%283%29)
![f(x) = (1-p)^{x}p](https://tex.z-dn.net/?f=f%28x%29%20%3D%20%281-p%29%5E%7Bx%7Dp)
![f(3) = (1-0.383)^{3}*(0.383)](https://tex.z-dn.net/?f=f%283%29%20%3D%20%281-0.383%29%5E%7B3%7D%2A%280.383%29)
![f(3) = 0.09](https://tex.z-dn.net/?f=f%283%29%20%3D%200.09)
There is a 9% probability that a drought lasts exactly 3 intervals.
At most 3 intervals?
This is ![P = f(0) + f(1) + f(2) + f(3)](https://tex.z-dn.net/?f=P%20%3D%20f%280%29%20%2B%20f%281%29%20%2B%20f%282%29%20%2B%20f%283%29)
![f(x) = (1-p)^{x}p](https://tex.z-dn.net/?f=f%28x%29%20%3D%20%281-p%29%5E%7Bx%7Dp)
![f(0) = (1-0.383)^{0}*(0.383) = 0.383](https://tex.z-dn.net/?f=f%280%29%20%3D%20%281-0.383%29%5E%7B0%7D%2A%280.383%29%20%3D%200.383)
![f(1) = (1-0.383)^{1}*(0.383) = 0.236](https://tex.z-dn.net/?f=f%281%29%20%3D%20%281-0.383%29%5E%7B1%7D%2A%280.383%29%20%3D%200.236)
![f(2) = (1-0.383)^{2}*(0.383) = 0.146](https://tex.z-dn.net/?f=f%282%29%20%3D%20%281-0.383%29%5E%7B2%7D%2A%280.383%29%20%3D%200.146)
Previously in this exercise, we found that ![f(3) = 0.09](https://tex.z-dn.net/?f=f%283%29%20%3D%200.09)
So
![P = f(0) + f(1) + f(2) + f(3) = 0.383 + 0.236 + 0.146 + 0.09 = 0.855](https://tex.z-dn.net/?f=P%20%3D%20f%280%29%20%2B%20f%281%29%20%2B%20f%282%29%20%2B%20f%283%29%20%3D%200.383%20%2B%200.236%20%2B%200.146%20%2B%200.09%20%3D%200.855)
There is an 85.5% probability that a drought lasts at most 3 intervals.
(b) What is the probability that the length of a drought exceeds its mean value by at least one standard deviation?
This is
.
We are working with discrete data, so 3.66 is rounded up to 4.
Either a drought lasts at least four months, or it lasts at most thee. In a), we found that the probability that it lasts at most 3 months is 0.855. The sum of these probabilities is decimal 1. So:
![P(X \leq 3) + P(X \geq 4) = 1](https://tex.z-dn.net/?f=P%28X%20%5Cleq%203%29%20%2B%20P%28X%20%5Cgeq%204%29%20%3D%201)
![0.855 + P(X \geq 4) = 1](https://tex.z-dn.net/?f=0.855%20%2B%20P%28X%20%5Cgeq%204%29%20%3D%201)
![P(X \geq 4) = 0.145](https://tex.z-dn.net/?f=P%28X%20%5Cgeq%204%29%20%3D%200.145)
There is a 14.5% probability that the length of a drought exceeds its mean value by at least one standard deviation