= Selecting appropriate distributions for constructing confidence... Raul You would like to construct a 90% confidence interval to estimate the population mean PCB (Polychlorinated biphenyl) level In white croaker fish from the San Francisco Bay. The random sample we select has a mean of 638 parts per million and a standard deviation of 50 parts per million. (a) What is the best point estimate, based on the sample, to use for the population mean? X 5 ? parts per million (b) For each of the following sampling scenarios, determine which distribution should be used to calculate the critical value for the 90% confidence interval for the population mean. (In the table, Z refers to a standard normal distribution, and t refers to a [ distribution.) Could use Sampling scenario Z t Unclear either Z or t X 5 ? The sample has size 16, and it is from a normally distributed population O O O O with a known standard deviation of 51. The sample has size 17, and it is from a normally distributed population O O O O with an unknown standard deviation. The sample has size 105, and it is from a non-normally distributed O O O O nanulation