Introduction to probability theory

Transcription

1 Introduction to probability theory Fátima Sánchez Cabo Institute for Genomics and Bioinformatics, TUGraz 07/03/ p. 1/35

2 Outline Random and conditional probability (7 March) From Probability Theory to Statistics (14 March) Hypothesis testing (14 March) Stochastic processes and Markov chains (21 March) 07/03/ p. 2/35

3 Introduction Sample space and events Probability Space Algebra of Sets Probability measure 07/03/ p. 3/35

4 Introduction Introduction Sample space and events Probability Space Algebra of Sets Probability measure Life is full of unpredictable events Probabilistic models are used to make inference from this type of : their outcome cannot be completely determined, but we can get some hints about the most likely situation that will occur Randomness does not mean chaos: for example the most likely value in a normally distributed random variable is the mode; very small or very large values are very unlikely to occur 0.4 Normal(0,1) /03/ p. 4/35

5 Random : Definition Introduction Sample space and events Probability Space Algebra of Sets Probability measure All outcomes of the experiment are known in advance But, it is a priori unknown which will be the outcome of each performance of the experiment: Systematic and random errors Complex processes, result of many combined processes The experiment can be repeated under identical conditions 07/03/ p. 5/35

6 Examples Introduction Sample space and events Probability Space Algebra of Sets Probability measure Throwing a die: Possible outcomes: {1,2,3,4,5,6} It is unknown what we will get next if we throw the die We can throw the die n times and all are independent trials Tossing a coin: Possible outcomes: {head, tail} It is unknown what we will get next if we toss the coin We can toss the coin n independent times The length life of a light bulb produced by a manufacturer: Possible outcomes: Any number between 0 and The life-time of a bulb is not known before hand We assume that the life-time of n bulbs can be measured under the same conditions 07/03/ p. 6/35

7 Sample space, events, σ algebra Introduction Sample space and events Probability Space Algebra of Sets Probability measure Sample space: Collection of possible elementary outcomes from a random experiment. 1. Throwing a die: Ω={1,2,3,4,5,6} 2. Tossing a coin: Ω={head, tail} 3. Life-time of a bulb: Ω = [0, ) Event: A set of outcomes of the experiment. 1. Throwing a die: "To obtain a 6" ({6}) or "Not to obtain a 6" ({1,2,3,4,5}) 2. Tossing a coin: "To obtain a head" 3. Life-time of a bulb: "The life-time of the bulb is greater than 3 years" A σ-field (or σ-algebra) is a non-empty collection of subsets of Ω that satisfy: ǫ F if A ǫ F then A c ǫ F,and if A i ǫ F is a countable sequence of sets then i A i ǫ F 07/03/ p. 7/35

8 Probability space Introduction Sample space and events Probability Space Algebra of Sets Probability measure The pair (Ω, F) is called under the previous conditions a sample space A measure is a nonnegative countably additive set function, i.e. a function µ : F R such as: 1. µ(a) µ( ) for all AǫF, and 2. if A i ǫ F and i A i = then µ( i A i) = i µ(a i) a If µ(ω) = 1 the measure is called a probability measure a countable: the set of the natural number, uncountable: the set of the real numbers 07/03/ p. 8/35

9 A little on Algebra of Sets Introduction Sample space and events Probability Space Algebra of Sets Probability measure Sets algebra is equivalent to elementary algebra (arithmetic): Intersection Multiplication; Union Addition Differently from arithmetic operations (only distributive for the product) union and intersection have both the distributive property Uniqueness of complements: Given A, B such as A B = Ω and A B =, then B = A c B\A = B A c Morgan s Laws: (A B) c = A c B c (A B) c = A c B c A cc = A c = Ω; Ω c = 07/03/ p. 9/35

10 Probability measure Introduction Sample space and events Probability Space Algebra of Sets Probability measure Some properties of the probability measure: 1. P(A B) = P(A) + P(B) P(A B) 2. P(A c ) = 1 P(A) 3. If B A P(B) = P(A) + P(B\A) P(A) 4. In general, if A 1,...,A n ǫf P( A i ) = = P(A i ) i<j P(A i A j )... + ( 1) n+1 P(A 1... A n ) Exercise: Proof /03/ p. 10/35

11 Example Introduction Sample space and events Probability Space Algebra of Sets Probability measure The probability of a chicken from Steiermark to be infected with the Bird flu is 0.2 a. At the same time a chicken may have another lethal disease called Y that appears with probability 0.5. Calculate the probability of chicken dying due to any disease: 1. If it is not possible that both diseases appear simultaneously; 2. If the probability of having both diseases is 0.6. a Unreal number 07/03/ p. 11/35

12 Dependent events Bayes Theorem 07/03/ p. 12/35

13 Dependent events Dependent events Bayes Theorem P(B)=2/3 P(W)=1/3 P(B)=1/3 P(W)=2/3 Which is the probability of getting a white ball? Is the same in both? 07/03/ p. 13/35

14 Dependent events Dependent events Bayes Theorem Definition: Two events A, B are called independent if P(A B) = P(A) P(B). Otherwise, P(A B) = P(A) P(B A) = P(B) P(A B) Multiplication rule: Given a probability space (Ω, F, P) such as A 1,...,A n ǫf and P( n 1 i=1 A i) > 0, then P( n i=1a i ) = P(A 1 )P(A 2 A 1 )...P(A n n 1 i=1 A i) 07/03/ p. 14/35

15 Dependent events Bayes Theorem Definition: (Ω, F, P) is a probability space such as the events A and B ǫ F. If P(B) > 0 it can be defined the probability of A conditional on B as: P(A B) = P(A B) P(B) Total probability theorem: Let Ω be the sample space of a random experiment and {A i, i = 1, 2,...} ǫ F such as A i Aj = i j and i A i = Ω. Then, for all B ǫ F P(B) = i P(B A i )P(A i ) 07/03/ p. 15/35

16 Example Dependent events Bayes Theorem The probability of a chicken from Steiermark to be infected with the Bird flu is 0.2 a. At the same time a chicken may have another lethal disease called Y that appears with probability 0.5. The death rate if a chicken has the Bird Flu is 0.8. The probability of death for a chicken with disease Y is 0.1. Both diseases cannot appear simultaneously. Additionally, a chicken might die due to natural causes with a probability 0.1. Calculate the probability of dying for a chicken in Steiermark. a Unreal number 07/03/ p. 16/35

17 Bayes Theorem Dependent events Bayes Theorem Let Ω be the sample space of a random experiment and {A i, i = 1, 2,...} ǫ F such as A i Aj = i j and i A i = Ω. Let B ǫ F with P(B)>0. Then P(A i B) = P(B A i) P(B) Equivalently, P(A i B) = P(B A i)p(a i ) j P(B A j)p(a j ) 07/03/ p. 17/35

18 Example Dependent events 1 Door A Door B Door C Bayes Theorem /03/ p. 18/35

19 Example 1 Door A Door B Door C Dependent events 2 3 Bayes Theorem Naive approach: Regardless to the initial situation now there are only two doors from which I could choose. Hence, Pr(car is behind A)=Pr(car is not behind A)= 1 2 It is not an advantage to switch the door. By Bayes Theorem: 07/03/ p. 19/35

20 Example (cont.) We define the event A as "car is behind door A" (the same for other doors) Dependent events Bayes Theorem Prior : Pr(A) = Pr(B) = Pr(C) = 1, i = 1, 2, 3 3 Pr(open C A) = 1 2 Pr(open C B) = 1 Pr(open C C) = 0 By the Total Probability Theorem: Pr(op C) = = P(op C A) P(A) + P(op C B) P(B) + P(op C C) P(C) = = /03/ p. 20/35

21 Example (cont.) By Bayes Theorem: Dependent events Bayes Theorem Pr(A open C) = Pr(B open C) = Pr(open C A) Pr(A) Pr(open C) Pr(open C B) Pr(B) Pr(open C) = = = 1 3 = 2 3 Conclusion: The probability of winning the car is bigger if you change the door!!! 07/03/ p. 21/35

22 Random variable Continuous random variables Distribution function Expectation and variance Central Limit Theorem 07/03/ p. 22/35

23 Random variable Random variable Continuous random variables Distribution function Expectation and variance Central Limit Theorem The probability measure P is a set function and hence difficult to work with it. We define a random variable on a probability space as a real valued function X defined in Ω such as: X : Ω R and X 1 (B) = {ω : X(ω)ǫB}ǫF, for all Borel set B R In other words, if the inverse of any Borel subset of R (semi-open intervals) belongs to the σ-algebra Example: The random experiment "To toss a coin" can be represented with the random variable { 1 if Head( X 1 (1) = HeadǫF) X = 0 if Tail( X 1 (0) = TailǫF) 07/03/ p. 23/35

24 Example 3.1. Random variable Continuous random variables Distribution function Expectation and variance Central Limit Theorem A roulette wheel has 38 slots -18 red, 18 black and 2 green. A gambler bets 1 $ to red each time. We define the random variable X i as the monetary gain of the gambler at game i. (From Durret (1996)) Ω = {Red, Black, Green} Events: F = {{Red}, {Black}, {Green}, {Red, Black}, {Red, Green}, {Black, Green} {Red, Black, Green}, } P(Red) = = 9 19, P(no Red) = = Random variable: X = { +1 Red 1 Red 07/03/ p. 24/35

25 Discrete random variables Random variable Continuous random variables Distribution function Expectation and variance Central Limit Theorem A discrete random variable can take a countable number of predetermined values Example: "To toss a coin", "To throw a die", "Number of cars crossing a line during a certain time interval" Mass function: For discrete random variables, the mass function determines the probability of each element of the sample space. Example: for the random experiment "To throw a die" p(1) =... = p(6) = Mass probability function of 100 random numbers from a binomial distribution (n=100, p=0.5) 07/03/ p. 25/35

26 Discrete random variables Typical mass functions of discrete random variables: Random variable Continuous random variables Distribution function Expectation and variance Central Limit Theorem 1. Bernoulli: X = { 1 p 0 (1 p) 2. Binomial: Number of successes within n Bernoulli trials: P(X = k) = C(n, k)p k (1 p) n k 3. Poisson, P(X = k) = e λ λ k k! 07/03/ p. 26/35

27 Continuous random variables Random variable Continuous random variables Distribution function Expectation and variance Central Limit Theorem For continuous random variables (that can take any real value) the way the probability is distributed within the sample space is more difficult to define. The probability density function (pdf) has the following properties: 1. P[a X b] = b a f(x)dx 2. f(x) 0, xǫr 3. f(x)dx = Histogram of frequencies for a normal random variable 07/03/ p. 27/35

28 Continuous random variables Most usual pdf Random variable Continuous random variables Distribution function Expectation and variance Central Limit Theorem 1. Normal distribution f(x) = 1 2πσ 2 exp{ (x µ)2 2σ 2 } Random errors are normally distributed 2. Uniform f(x) = 1 b a I [a,b] 3. Gamma f(x) = βα Γ(α) xα 1 e βx 07/03/ p. 28/35

29 Distribution function F(x) = P(X x) = { x f(x)dx cont r.v. a P(X = k) discrete r.v. x a Random variable Continuous random variables Distribution function Expectation and variance Central Limit Theorem where a is the smallest value that the r.v. can take. Empirical CDF Empirical CDF F(x) 0.5 F(x) x x Probability distribution of 100 random numbers from a binomial distribution (n=10, p=0.5) and cumulative probability distribution of 1000 random numbers from a normal distribution (µ=0, σ 2 =1) 07/03/ p. 29/35

30 Example 3.2. Random variable Continuous random variables Distribution function Expectation and variance Central Limit Theorem A fair coin (p=0.5) is tossed twice. For each one of the possible outcomes of the experiment we define the random variable that indicates the number of heads, i.e.: X = 2 HH 1 HT,TH 0 TT Calculate and plot the probability distribution for this random variable. 07/03/ p. 30/35

31 Distribution function and probability Properties of the distribution function: Random variable Continuous random variables Distribution function Expectation and variance Central Limit Theorem 1. lim x = 0; lim x + = 1 2. If x < y F(x) F(y) 3. F is continuous from the right, i.e. F(x + h) F(x) as h 0 Distribution function and probability: P(X > x) = 1 F(x) P(x < X y) = F(y) F(x) 07/03/ p. 31/35

32 Expectation and variance Random variable Continuous random variables Distribution function Expectation and variance Central Limit Theorem Expectation: Continuous random variable: E[X] = xf(x)dx Discrete random variable: E[X] = x i P(X = x i ) Variance:V [X] = E[(X E(X)) 2 ] = E[X 2 ] (E[X]) 2 Probability Statistic Base Population Sample Central tendency Expectation Average Dispersion Variance Sample variance 07/03/ p. 32/35

33 Central Limit Theorem Random variable Continuous random variables Distribution function Expectation and variance Central Limit Theorem Given X 1,...,X n a set of random variables independents and with common distribution f, such as E[X i ] = µ and V [X i ] = σ 2 for all i. Then it is true that: If n is big enough. X E[ X] V [ X] N(0, 1) 07/03/ p. 33/35

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35 [1] Durbin, R., Eddy, S., Krogh, A. and Mitchison, G. (1996) Biological sequence analysis, Cambridge University Press. [2] Durret, R. (1996) Probability: Theory and examples, Duxbury Press, Second edition. [3] Rohatgi, V.K. and Ehsanes Saleh, A.K.Md. (1988) An introduction to probability and statistics, Wiley, Second Edition. [4] Tuckwell, H.C. (1988) Elementary applications of probability theory, Chapman and Hall. [5] tsirel/courses/introprob/syl1a.html [Engineering statistics] 07/03/ p. 35/35