Question 1 of 12 When estimating a population mean, you must use t procedures rather than normal curve procedures when: A. the sample standard deviation is unknown. O B. the population standard deviation and the sample standard deviation are different. O C. the population standard deviation is unknown. O) D. the sample size is less than 15, but not when it's at least 15. O E. the standard deviation is less than 10.Question 2 of 12 We say that the f procedures are robust because: O A. they don't require random samples. O B. they can be used even if some of the assumptions for their use are violated, O C. they depend on degrees of freedom. O) D. they're so strong they can be used in many different situations. O)E. they can be used even when the sample size is quite small.Question 3 of 12 Say you're going to conduct a f procedure for the difference between the means of two populations. You decide the samples, of sizes 12 and 15, are independent. Which of the following statements are true? I. If we pool variances, the number of degrees of freedom is 25. Il. If we don't pool variances, and we don't have access to software to compute degrees of freedom, the number of degrees of freedom is 11. Ill. For a given level of confidence C, a confidence interval based on 11 degrees of freedom will be wider than one based on 25 degrees of freedom. O A. I only O B. II only O C. Ill only O D. I and II only O E. I, II, and IllQuestion 4 of 12 What's the critical value of : (r) needed to construct a 98% confidence interval for the mean of a distribution based on a sample of size 22? A. 2.508 O B. 2.518 O C. 2189 O D. 2.183 O E. 2.500Question 5 of 12 What's the critical value of f necessary to construct a 90% confidence interval for the difference between the means of two distinct populations of sizes 7 and 87 (Assume that the conditions necessary to justify pooling variances have been met.) O A. 1.943 O B. 1.761 O C. 1,753 O D. 1.895 O E. 1.771