Exam 8: Interval Estimation

The manager of a department store wants to determine what proportion of people who enter the store use the store's credit cards for their purchases. What size sample should he take so that at 95% confidence the error will not be more than 6%?

A statistician employed by a consumer testing organization reports that at 95% confidence he has determined that the true average content of the Uncola soft drinks is between 11.7 to 12.3 ounces. He further reports that his sample revealed an average content of 12 ounces, but he forgot to report the size of the sample he had selected. Assuming the standard deviation of the population is 1.28, determine the size of the sample.

As the number of degrees of freedom for a t distribution increases, the difference between the t distribution and the standard normal distribution

A sample of 36 patients in a doctor's office showed that they had to wait an average of 45 minutes with a standard deviation of 10 minutes before they could see the doctor. Provide a 90% confidence interval estimate for the average waiting time of all the patients who visit this doctor.

Exhibit 8-5 A random sample of 64 SAT scores of students applying for merit scholarships showed an average of 1400 with a standard deviation of 240. -Refer to Exhibit 8-5. The 95% confidence interval for the SAT scores is

Two hundred students are enrolled in an Economics class. After the first examination, a random sample of 6 papers was selected. The grades were 65, 75, 89, 71, 70 and 80. a. Determine the standard error of the mean. b. What assumption must be made before we can determine an interval for the mean grade of all the students in the class? Explain why. c. Assume the assumption of Part b is met. Provide a 95% confidence interval for the mean grade of all the students in the class.

The following random sample from a population whose values were normally distributed was collected. 10 12 18 16 The 80% confidence interval for μ is

A random sample of 49 statistics examinations was taken. The average score, in the sample, was 84 with a variance of 12.25. The 95% confidence interval for the average examination score of the population of the examinations is