) In a rural area, the infected rate of certain disease is 0.20 (i.e., 20%). A sample of 20 people is taken from the area - selections are independent. Define X as the number of people in the sample who are infected by the disease.

(1) This X is a binomial variable. What are the parameter values of this distribution? What are the mean value and standard deviation of X?

(2) What is the probability that at least 4 of the 20 people are infected? Use Binomial table.

(3) Compute P(X  4) using normal approximation and Poisson approximation, respectively.

Normal approximation:

Poisson approximation:

(4) Are those in (3) good approximations? Why?

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asked by Trell

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