Multivariate Probability Distributions
Multivariate Random Variables In many settings, we are interested in 2 or more characteristics observed in experiments Often used to study the relationship among characteristics and the prediction of one based on the other(s) Three types of distributions: Joint: Distribution of outcomes across all combinations of variables levels Marginal: Distribution of outcomes for a single variable Conditional: Distribution of outcomes for a single variable, given the level(s) of the other variable(s)
Joint Distribution
Marginal Distributions
Conditional Distributions Describes the behavior of one variable, given level(s) of other variable(s)
Expectations
Expectations of Linear Functions
Variances of Linear Functions
Covariance of Two Linear Functions
Multinomial Distribution Extension of Binomial Distribution to experiments where each trial can end in exactly one of k categories n independent trials Probability a trial results in category i is pi Yi is the number of trials resulting in category I p1+…+pk = 1 Y1+…+Yk = n
Multinomial Distribution
Multinomial Distribution
Conditional Expectations When E[Y1|y2] is a function of y2, function is called the regression of Y1 on Y2
Unconditional and Conditional Mean
Unconditional and Conditional Variance
Compounding Some situations in theory and in practice have a model where a parameter is a random variable Defect Rate (P) varies from day to day, and we count the number of sampled defectives each day (Y) Pi ~Beta(a,b) Yi |Pi ~Bin(n,Pi) Numbers of customers arriving at store (A) varies from day to day, and we may measure the total sales (Y) each day Ai ~ Poisson(l) Yi|Ai ~ Bin(Ai,p)