8. In interval estimation, the t distribution is applicable only when a. the population has a mean of less than 30. b. the sample standard deviation (s) is given instead of the population standard deviation (o). c. the variance of the population is known. d. the standard deviation of the population is known. e. we will always use the t distribution. 9. To assess the precision of a laboratory scale, we measure a block known to have a mass of 1 gram. We measure the block n times and record the mean of the measurements. Suppose the scale readings are normally distributed with unknown mean u and standard deviation - 0.001 g. How large should n be so that a 95% confidence interval for u has a margin of error of 0.0001? a. 20 b. 385 C. 10,000 d. 66,358 c. 384 10. A 95% confidence interval for the mean reading achievement score for a population of third grade students is (44.2, 54.2). Suppose you compute a 99% confidence interval using the same information. Which of the following statements is correct? a. The intervals have the same width. b. The 99% interval is longer. C. The 99% interval is shorter. d. None of the above.