12a, 12b, 13, 14a, 14b

12. Determine whether the t distribution should be used in each situation: a) A simple random sample of size 8 is drawn from a distribution that is approximately normal. b) A simple random sample size of 15 is drawn from a distribution that is not close to normal. 13. A company has developed a new type of lightbulb, and wants to estimate its mean lifetime. A simple random sample of 100 bulbs had a mean lifetime of 750.2 hours with sample standard deviation of 30 hours. Construct a 95% confidence interval for the population mean lifetime of all bulbs manufactured. 14. A sample of n = 35 has sample mean of 34.85 and sample standard deviation of17.9 a) Construct a 98% confidence interval for the population mean. b) If the confidence level were 95%, would the confidence interval be narrower or wider