Part II: Discrete Random Variables

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Part II: Discrete Random Variables http://neveryetmelted.com/categories/mathematics/

Chapter 6: Random Variables; Discrete Versus Continuous http://math.sfsu.edu/beck/quotes.html

Chapter 7: Probability Mass Functions and Cumulative Distribution Functions The 50-50-90 rule: Anytime You have a 50-50 chance of getting something right, there’s a 90% probability you’ll get it wrong. Andy Rooney http://brownsharpie.courtneygibbons.org/?p=161

Probability Mass Density Cumulative Density Function Def. 7.1: If X is a random variable, the probability that X is exactly equal to X is called the PMF pX(x) = P(X = x) Def. 7.2: If X is a random variable, the probability that X does not exceed x is called the CDF FX(x) = P(X ≤ x)

Histogram: interpretation of PMF Simulated 1000 times Theoretical Simulated 10,000 times

Example 7.4 (Fig. 7.3) mass CDF

More on CDFs CDFs should always be written as piece-wise functions like the following which is from Problem 2 on the handout 𝐹 𝑋 𝑥 = 0 𝑥≤0 0.369 0≤𝑥<1 0.584 0.816 1 1≤𝑥<2 2≤𝑥<3 03≤𝑥

More on CDFs (cont.) Because CDFs are not right continuous, P(X ≤ a) ≠ P(X < a) FX(a) = P(X ≤ a) FX(a-) = P(X < a) To calculate a probability P(a < X ≤ b) = FX(b) – FX(a) P(a ≤ X < b) = FX(b-) – FX(a-) a b a b

Calculation of Probabilities from CDFs Let X be a random variable. Then for all real numbers a,b where a < b P(a < X ≤ b) = FX(b) – FX(a) P(a ≤ X ≤ b) = FX(b) – FX(a-) P(a < X < b) = FX(b-) – FX(a) P(a ≤ X < b) = FX(b-) – FX(a-) P(X = a) = FX(a) – FX(a-)

Chapter 8: Jointly Distributed Random Variables; Independence and Conditioning The most misleading assumptions are the ones you don’t even know you’re making. Douglas Adams and Mark Carwardine http://math.stackexchange.com/questions/314072/joint-probability-mass-function X

Chapters 9: Expected Values of Discrete Random Variables http://www.cartoonstock.com/directory/a/average_family_gifts.asp X

Chapter 10: Expected Values of Sums of Random Variables http://faculty.wiu.edu/JR-Olsen/wiu/stu/m206/front.htm X

Chapter 11: Expected Values of Functions of Discrete Random Variables; Variance of Discrete Random Variables http://fightingdarwin.blogspot.com/2011_12_01_archive.html X