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 June 7th, 2013, 10:14 AM #1 Senior Member   Joined: Jul 2012 Posts: 225 Thanks: 0 What am I doing wrong? simple probability Here's the question: X and Y are discrete random variables that get values from 1 to N with equal probability. In other words, X and Y have a uniform distribution with values between 1 and N. We defined another discrete random variable Z. Z=X-Y. Find the probability function of Z. My answer is incorrect, but I don't know why and I'd like an explanation. Here is my answer: Z can get these values: We can see that there are N-k events that yield X-Y=k, out of overall possible events. This gives us that But that is an incorrect answer. Why? June 7th, 2013, 02:11 PM   #2
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Re: What am I doing wrong? simple probability

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 Originally Posted by OriaG Here's the question: X and Y are discrete random variables that get values from 1 to N with equal probability. In other words, X and Y have a uniform distribution with values between 1 and N. We defined another discrete random variable Z. Z=X-Y. Find the probability function of Z. My answer is incorrect, but I don't know why and I'd like an explanation. Here is my answer: Z can get these values: We can see that there are N-k events that yield X-Y=k, out of overall possible events. This gives us that But that is an incorrect answer. Why?
When k < 0, you need to check your count.
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