Exam 8: Interval Estimation

Exhibit 8-3 The manager of a grocery store has taken a random sample of 100 customers.The average length of time it took these 100 customers to check out was 3.0 minutes.It is known that the standard deviation of the population of checkout times is one minute. -Refer to Exhibit 8-3.The 95% confidence interval for the true average checkout time (in minutes)is

If an interval estimate is said to be constructed at the 90% confidence level,the confidence coefficient would be

A simple random sample of 36 items resulted in a sample mean of 40 and a standard deviation of 12.Construct a 95% confidence interval for the population mean.

When constructing a confidence interval for the population mean and the standard deviation of the sample is used,the degrees of freedom for the t distribution equals

A local university administers a comprehensive examination to the candidates for B.S.degrees in Business Administration.Five examinations are selected at random and scored.The scores are shown below. a.Compute the mean and the standard deviation of the sample. b.Compute the margin of error at 95% confidence. c.Develop a 95% confidence interval estimate for the mean of the population.Assume the population is normally distributed.

From a population with a variance of 900,a sample of 225 items is selected.At 95% confidence,the margin of error is

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.If we want to provide a 95% confidence interval for the SAT scores,the degrees of freedom for reading the critical values of "t" statistic is

The monthly starting salaries of students who receive an MBA degree have a population standard deviation of $110.What size sample should be selected to obtain a 0.95 probability of estimating the mean monthly income within $20 or less?

If the population standard deviation of the lifetime of washing machines is estimated to be 900 hours,how large a sample must be taken in order to be 97% confident that the margin of error will not exceed 100 hours?

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%?

In order to determine how many hours per week freshmen college students watch television,a random sample of 256 students was selected.It was determined that the students in the sample spent an average of 14 hours with a standard deviation of 3.2 hours watching TV per week. a.Provide a 95% confidence interval estimate for the average number of hours that all college freshmen spend watching TV per week. b.Assume that a sample of 62 students was selected (with the same mean and the standard deviation).Provide a 95% confidence interval estimate for the average number of hours that all college freshmen spend watching TV per week.

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

Information regarding the price of a roll of camera film (35 mm,24 exposure)for a sample of 12 cities worldwide is shown below.Determine a 95% confidence interval for the population mean.

When the level of confidence decreases,the margin of error

It is known that the population variance equals 484.With a 0.95 probability,the sample size that needs to be taken if the desired margin of error is 5 or less is

In order to estimate the average electric usage per month,a sample of 196 houses was selected,and their electric usage determined. a.Assume a population standard deviation of 350-kilowatt hours.Determine the standard error of the mean. b.With a 0.95 probability,determine the margin of error. c.If the sample mean is 2,000 KWH,what is the 95% confidence interval estimate of the population mean?

The average monthly electric bill of a random sample of 256 residents of a city is $90 with a standard deviation of $24. a.Construct a 90% confidence interval for the mean monthly electric bills of all residents. b.Construct a 95% confidence interval for the mean monthly electric bills of all residents.

A random sample of 100 credit sales in a department store showed an average sale of $120.00.From past data,it is known that the standard deviation of the population is $40.00. a.Determine the standard error of the mean. b.With a 0.95 probability,determine the margin of error. c.What is the 95% confidence interval of the population mean?

The sample size needed to provide a margin of error of 2 or less with a .95 probability when the population standard deviation equals 11 is

A random sample of 87 airline pilots had an average yearly income of $99,400 with a standard deviation of $12,000. a.If we want to determine a 95% confidence interval for the average yearly income,what is the value of t? b.Develop a 95% confidence interval for the average yearly income of all pilots.