Distribution of Sum of Two Weird Random Variables....

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Discussion Overview

The discussion revolves around determining the distribution of a random variable, x, defined as a linear combination of two other random variables, z and y, which are distributed normally and exponentially, respectively. The conversation explores the implications of their independence and the methods for calculating the distribution of x through convolution and integration.

Discussion Character

  • Exploratory, Technical explanation, Mathematical reasoning

Main Points Raised

  • One participant proposes that if z and y are independent, the distribution of x can be found by considering the sum of two random variables, r_1 = b*z and r_2 = c*y.
  • Another participant suggests using convolution to find the distribution of x, stating that it can be expressed as an integral involving the distributions of r_1 and r_2.
  • A further contribution discusses the conditional probabilities involved when calculating the probability of z and y given a specific value of x, emphasizing the need to integrate over z.
  • Another participant notes the limitations of the ranges of z and y, particularly that y, being exponential, can only take nonnegative values, which affects the integration limits for z.
  • One participant mentions the concept of Propagation of Error/Uncertainty as a broader topic related to determining the (co)variance of functions of random variables.

Areas of Agreement / Disagreement

Participants express various approaches to the problem, with no consensus reached on a single method or solution. Multiple competing views on how to handle the distributions and their properties remain present.

Contextual Notes

There are unresolved assumptions regarding the independence of the random variables and the specific conditions under which the proposed methods apply. The discussion also highlights the need for careful consideration of the ranges of the random variables involved.

Steve Zissou
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Hi there.

Let's say I have the following relationship:

x = a + b*z + c*y

z is distributed normally
y is distributed according to a different distribution, say exponential

Is there a way to figure out what is the distribution of x?

Thanks!
 
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I will assume that the two random variables are independent.
The constant offset, ##a##, can always be dealt with last. So let's ignore it for now.
Consider the sum of two random variables, ##r_1 = b*z## and ##r_2 = c*y## with distributions ##p_1(r_1)## and ##p_2(r_2)##, respectively.
The distribution, ##p(x)##, of the sum, ##x = r_1+r_2## is the convolution, ##p(x) = \int_{t=-\infty}^{t=\infty}p_1(t)p_2(x-t) \,dt ##
 
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Nice! Thank you very much, FactChecker!
 
For a given value x = X,
What is the probability that z has a value Z?
What is the probability that y has the value Y = (X -a -bZ)/c?
Integrate over z: f(x=X) = ∫f(z=Z)f(Y=(X-a-bZ)/c)dZ
(Assuming z and y are distributed independently. If not, you have to use a conditional probability for Y.)
Note that the range of z and y may be limited to less than their full possible range, e.g. if z is normal, it can take negative values, but if y is exponential it can only be positive (or zero). Therefore Z is limited to values for which X - a - bZ is nonnegative, ie Z ≤ (X - a)/b. (That's if b and c are both positive, work it out for yourself for other cases.)
 
Thanks, mjc123!
 
Thought I would generalize by referring to the topic of Propagation of Error/Uncertainty, who's topic is to determine the (co)variance of functions of Random Variables.
 

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