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STA291 Statistical Methods Lecture 13

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Last time … Notions of: o Random variable, its o expected value, o variance, and o standard deviation 2

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Bernoulli Distribution o Suppose we have a single random experiment X with two outcomes: “success” and “failure.” o Typically, we denote “success” by the value 1 and “failure” by the value 0. o It is also customary to label the corresponding probabilities as: P (success) = P ( 1 ) = p and P (failure) = P ( 0 ) = 1 – p = q o Note: p + q = 1 o What are expected value and variance of a Bernoulli random variable?

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Binomial Distribution I o Suppose that instead of just one Bernoulli experiment, we perform several and they are all independent of each other. o Let’s say we do n of them. The value n is the number of trials. o We will label these n Bernoulli random variables in this manner: X 1, X 2, …, X n o As before, we will assume that the probability of success in a single trial is p, and that this probability of success doesn’t change from trial to trial.

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Binomial Distribution II o Now, we will build a new random variable X using all of these Bernoulli random variables: o What are the possible outcomes of X ? o What is X counting? o How can we find P ( X = k )?

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Binomial Distribution III o We need a quick way to count the number of ways in which k successes can occur in n trials. o Here’s the formula to find this value: o Note: n C k is read as “ n choose k.”

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Binomial Distribution IV o Now, we can write the formula for the binomial distribution: o The probability of observing k successes in n independent trials is o under the assumption that the probability of success in a single trial is p.

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Using Excel Helpful resource (besides your calculator): Excel: EnterGives =BINOMDIST(4,10,0.2,FALSE)0.08808 =BINOMDIST(4,10,0.2,TRUE)0.967207

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Center and Spread of a Binomial Distribution Unlike generic distributions, you don’t need to go through using the ugly formulas to get the mean, variance, and standard deviation for a binomial random variable (although you’d get the same answer if you did):

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Looking back o Binomial distribution: o Criteria o Calculation of probabilities o Mean, variance, standard deviation o Interpretation of result o Transition to … ?

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