* You wanted to estimate the mean number of vehicles crossing abusy bridge in your neighborhood each morning during rush hour forthe past year. To accomplish this, you stationed yourself and a fewassistants at one end of the bridge on 31 randomly selectedmornings during the year and counted the number of vehiclescrossing the bridge in a 10-minute period during rush hour. Youfound the mean to be 119 vehicles per minute, with a standarddeviation of 31.

(a) Construct the 95% confidence interval for thepopulation mean (vehicles per minute).

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(b) Construct the 99% confidence interval for thepopulation mean (vehicles per minute)

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* A college counselor wants to determine the average amount oftime first-year students spend studying. He randomly samples 31students from the freshman class and asks them how many hours aweek they study. The mean of the resulting scores is 17 hours, andthe standard deviation is 5.8 hours.

(a) Construct the 95% confidence interval for thepopulation mean.
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(b) Construct the 99% confidence interval for thepopulation mean.

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*A cognitive psychologist believes that a particular drugimproves short-term memory. The drug is safe, with no side effects.An experiment is conducted in which 8 randomly selected subjectsare given the drug and then given a short time to memorize a listof 10 words. The subjects are then tested for retention 15 minutesafter the memorization period. The number of words correctlyrecalled by each subject is as follows: 6, 11, 12, 4, 6, 7, 8, 6.Over the past few years, the psychologist has collected a lot ofdata using this task with similar subjects. Although he has lostthe original data, he remembers that the mean was 4.9 wordscorrectly recalled and that the data were normally distributed.

(a) On the basis of these data, what can we concludeabout the effect of the drug on short-term memory? Use α =0.05_{2 tail} in making your decision.

tobt =

t_crit = ±