## Description

STAT 230

Assignment 3

Full Name: Out of 25

Instructions:

You must show significant steps to get full marks!

This assignment is out of 25 points.

1. Formally prove the following:

(a) (2 points) Let X be a continuous random variable that is normally distributed with mean µ and standard deviation σ. Show that approximately 95% of the area under the normal density curve is within 2 standard deviations of the mean.

(b) (2 points) Let Y1 and Y2 be uncorrelated (independend) random variables and let U1 = Y1 + Y2 and U2 = Y1 − Y2. Find Cov(Y1,Y2) in terms of the variance of Y1 and Y2.

2. (4 points) Given the following probability density function,

0 ≤ x ≤ 1,0 ≤ y ≤ 1 elsewhere

Find the covariance of X and Y .

3. (4 points) Let X and Y be discrete random variables with joint probability distribution shown below.

x

y −1 0 1

−1 1

16 3

16 1

16

0 3

16 0 3

16

1 1

16 3

16 1

16

Show that X and Y are dependent, but have zero covariance.

4. A chemical process has produced, on the average, 600 tons of chemical per day. The daily yields for the past week are 585, 604, 590, 593, 602, 598 and 593 tons.

(a) (6 points) Do these data indicate that the average yield is less than 600 tons and hence that something is wrong with the process? Test at the 5% level of significance.

(b) (1 point) What assumptions are required for the valid use of the procedure you used to analyze these data.

(c) (1 point) Would you decision change if the level of significance was α = 0.01? Explain.

(d) (1 point) Would you decision change if the alternative hypothesis was two-tailed with α = 0.05? Explain.

(e) (3 points) Construct a 95% confidence interval for the true mean weekly yield and interpret the interval.

(f) (1 point) Is the result consistent with your conclusion in part (a)? Explain.

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