# For a partition B 1,..., B n, where B i B j = for i. A = (A B 1 ) (A B 2 ),..., (A B n ) and thus. P (A) = P (A B i ) = P (A B i )P (B i )

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1 Probability Review Cynthia Rudin A probability space, defined by Kolmogorov ( ) consists of: A set of outcomes S, e.g., for the roll of a die, S = {1, 2, 3, 4, 5, 6}, for the roll of two dice, S =,,,,..., temperature on Monday, S = [ 50, 50]. A set of events, where an event is a subset of S, e.g., roll at least one 3 :,..., temperature above 80 : (80, 150] The union, intersection and complement of events are also events (an algebra). A probability measure, which gives a number between 0 and 1 to each event, where P (S) = 1 and A B = P (A B) = P (A) + P (B). Think of P as measuring the size of a set, or an area of the Venn diagram. The conditional probability of event A given event B is: P (A B) P (A B) :=. P (B) 1

2 Event A is independent of B if P (A B) = P (A). That is, knowing B occurred doesn t impact whether A occurred (e.g. A and B each represent an event where a coin returned heads). In that case, P (A B) P (A) = P (A B) := so P (A B) = P (A)P (B). P (B) Do not confuse independence with disjointness. Disjoint events A and B have P (A B) = 0. For a partition B 1,..., B n, where B i B j = for i and thus A = (A B 1 ) (A B 2 ),..., (A B n ) = j and B 1 B 2 B n = S then n P (A) = P (A B i ) = P (A B i )P (B i ) i=1 i from the definition of conditional probability. Bayes Theorem P (A B)P (B) P (B A) =. P (A) Derivation of Bayes Rule Monty Hall Problem A random variable (r.v.) assigns a number to each outcome in S. Example 1: toss 2 dice: random var. X is the sum of the numbers on the dice. Example 2: collections of transistors: random var X is the min of the lifetimes of the transistors. 2

3 A probability density function (pdf... or probability mass function pmf) for random variable X is defined via: f(x) = P (X = x) for discrete distributions (Note f(x) 0, x f(x) = 1.) b P (a X b) = a f(x)dx for continuous distributions. (Note f(x) 0, f(x) = 1.) The cumulative distribution function (cdf) for r.v. X is: F (x) = P (X x) = f(k) (discrete) k x x = f(y)dy (continuous) 3

4 The expected value (mean) of an r.v. X is: E(X) = µ = xf(x) (discrete) x Expectation is linear, meaning Roulette The variance of an r.v. X is: E(X) = µ = xf(x)dx (continuous). x E(aX + by ) = ae(x) + be(y ). V ar(x) = σ 2 = E(X µ) 2. Variance measures dispersion around the mean. Variance is not linear: The standard deviation (SD) is: V ar(ax + b) = a 2 V ar(x). SD(x) = σ = V ar(x). Note that people sometimes use another definition for variance that is equivalent: V ar(x) = σ 2 = E(X 2 ) (E(X)) 2. Sum of Two Dice Quantiles/Percentiles The p th quantile (or 100p th percentile), denoted θ p, of r.v. X obeys: P (X θ p ) = p. Note: 50 th percentile, or.5 th quantile, is the median θ.5. Exponential Distribution 4

5 Section 2.5. Jointly Distributed R.V. s Jointly distributed r.v. s have joint pdf s: f(x, y) = P (X = x, Y = y). Their covariance is defined by Cov(X, Y ) := σ x,y := E[(X µ x )(Y µ y )]. The first term considers dispersion from the mean of X, the second term is dispersion from the mean of Y. If X and Y are independent, then E(XY ) = E(X)E(Y ) = µ x µ y (see expected value on Wikipedia). So, multiplying the terms out and passing the expectation through, we get: Cov(X, Y ) = E[(X µ x )(Y µ y )] = E(XY ) µ x E(Y ) µ y E(X) + µ x µ y = 0. Useful relationships: 1. Cov(X, X) = E[(X µ x ) 2 ] = V ar(x) 2. Cov(aX + c, by + d) = ab Cov(X, Y ) 3. V ar(x±y ) = V ar(x)+v ar(y )±2Cov(X, Y ) where Cov(X, Y ) is 0 if X and Y are indep. The correlation coefficient is a normalized version of covariance so that its range is [-1,1]: Cov(X, Y ) σ XY ρ XY := Corr(X, Y ) := =. V ar(x)v ar(y ) σ X σ Y Section 2.6. Chebyshev s Inequality (we don t use it much in this class) σ 2 For c > 0, P ( X EX c). c 2 Chebyshev s inequality is useful when you want a upper bound on how often an r.v. is far from its mean. You can t use it for directly calculating P ( X EX c), only for bounding it. Weak Law of Large Numbers Let s say we want to know the average number of pizza slices we will sell in a day. We ll measure pizza sales over the next couple weeks. Each day gets a random variable X i which represents the amount of pizza we sell on day i. Each X i has the same distribution, because each day basically has the same kind of randomness as the others. Let s say we take the average of over the couple weeks we measured. There s a random variable for that, it s X = 1 n i X i. Does X have anything to do with the average pizza sales per day? In other words, does measuring X tell us anything about the X i s? For instance, (on average) is X close to the average sales per day, E(X i )? 5

6 Does it matter what the distribution of pizza sales is? For instance, what if we usually have 1-4 customers but sometimes we have a conference where we have people; that is kind of a weird distribution. Does that mean that the X i needs to be adjusted in some way to help us understand the X i s? It turns out the first answer is yes, in fact pretty often, the average X is very similar to the average value of X i. Especially when n is large. The second answer is also yes, but as long as n is large, X is very similar to the average value of X i. This means that no matter what the distribution is, as long as we measure enough days, we can get a pretty good sense of the average pizza sales. Weak LLN: Let X be a r.v. for the sample mean X = 1 n i X i of n iid (independent and identically distributed) r.v. s. The distribution for each X i has finite mean µ and variance 2 σ. Then for any c > 0, P ( X µ c) 0 as n. W eak LLN says that X approaches µ when n is large. Weak LLN is nice because it tells us that no matter what the distribution of the X i s is, the sample mean approaches the true mean. Proof Weak LLN using Chebyshev Section 2.7. Selected Discrete Distributions Bernoulli X Bernoulli(p) coin flipping f(x) = P (X = x) = { p if x = 1 heads 1 p if x = 0 tails Binomial X Bin(n, p) n coins flipping, balls in a bag drawn with replacement, balls in an infinite bag ( ) n f(x ) = P (X = x) = p x (1 p) n x for x = 0, 1,..., n, x where n is the number of ways to distribute x successes and n x failures, x ( ) n n! =. x x!(n x)! (If you have n coins flipping, f(x) is the probability to get x heads, p is the probability of heads.) X Bin(n, p) has E(X) = np, V ar(x) = np(1 p). You ll end up needing these facts in Chapter 9. 6

7 Hypergeometric X HyGE(N, M, n) balls in a bag drawn without replacement, where: N is the size of the total population (the number of balls in the bag), M is the number of items that have a specific attribute (perhaps M balls are red), n is our sample size, f(x) is the probability that x items from the sample have the attribute. ways to draw x balls ways to draw n x balls ( ) ( ) M N M f(x) = with attribute without attribute x n x = ( ) ways to draw n balls N. n Multinomial Distribution generalization of binomial Think of customers choosing backpacks of different colors. A random group of n customers each choose their favorite color backpack. There is a multinomial distribution governing how many backpacks of each color were chosen by the group. Below, x k is the number of people who ordered the k th color backpack. Also, f(x) is the probability that x 1 customers chose color 1, x 2 customers chose color 2, and so on. f(x 1, x 2, x 3,..., x k ) = P (X 1 = x 1, X 2 = x 2, X 3 = x 3,..., X k = x k ) n! x 1 x 2 x 3 x = p p p... p k k x 1!x 2!x 3!... x k! where x i 0 for all i and i x i = n (there are n total customers), and p i is the probability that outcome i occurs. (p i is the probability to choose the i th color backpack). 2.8 Selected Continuous Distributions Uniform Distribution X U[a, b] f(x) = { 1 a x b b a 0 otherwise Poisson Distribution X P ois(λ) binomial when np λ, rare events X P ois(λ) has e λ λ x f(x) = x = 0,1,2,... x! E(X) = λ, V ar(x) = λ. 7

8 Exponential Distribution X Exp(λ) waiting times for Poisson events f(x) = λe λx for x 0 Gamma Distribution X Gamma(λ, r) sums of r iid exponential r.v. s, sums of waiting times for Poisson events r 1 λx λ r x e f(x) = for x 0, Γ(r) where Γ(r) is the Gamma function, which is a generalization of factorial. Customers on line Normal (Gaussian) Distribution X N(µ, σ 2 ) We have that: 1 (x µ) 2 /2σ 2 f(x) = e for < x <. σ 2π E(X) = µ, V ar(x) = σ 2. We often standardize a normal distribution by shifting its mean to 0 and scaling its variance to 1: X µ If X N(µ, σ 2 ) then Z = N(0, 1) standard normal σ Since we can standardize any gaussian, let s work mostly with the standard normal Z N(0, 1): pdf: 1 φ(z) = e z2 /2 2π z cdf: Φ(z) = P (Z z) = φ(y)dy We can t integrate the cdf in closed form. This means that in order to get from z to Φ(z) (or the reverse) we need to get the answer from a book or computer where they integrated 8

9 it numerically, by computing an approximation to the integral. MATLAB uses the command normcdf. Table A.3 in your book has values for Φ(z) for many possible z s. Read the table left to right, and up to down. So if the entries in the table look like this: z This means that for z = 2.43, then Φ(z) = P (Z z) = So the table relates z to Φ(z). You can either be given z and need Φ(z) or vice versa. 75 th percentile calculation Denote z α as the solution to 1 Φ(z) = α. z α is called the upper α critical point or the 100(1 α) th percentile. Linear Combinations of Normal r.v. s are also normal. For n iid observations from N(µ, σ 2 ), that is. the sample mean X obeys: X i N(µ, σ 2 ) for i = 1,..., n, ( ) σ X 2 N µ,. n Hmm, where have you seen σ 2 /n before? Of course! You saw it here: V ar( X) = V ar 1 n i X i = 1 n 2 i V ar(x i ) = nσ2 n 2 = σ2 n. This is the variance of the mean of n iid r.v. s who each have V ar(x i ) = σ 2. 9

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