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# Confidence Interval

### 95 percent confidence interval

Of a random sample of 600 trucks at a bridge, 114 had bad signal lights. Construct a 95 percent confidence interval for the percentage of trucks that had bad signal lights.

### Determining a confidence interval

Computer Services, Inc., wants to determine a confidence interval for the average CPU time of their teleprocessing transactions. A sample of 64 transactions yielded a mean of 6 seconds with a standard deviation of 0.8 seconds. Determine a 98% confidence interval for the average CPU time.

### Confidence interval for mean ..

A random sample of 81 credit sales in a department store showed an average sale of \$68.00. Historically, it is known that the standard deviation of the population is \$27.00. a) determine the standard error of the mean, b) with a 0.95 probability, what can be said about the margin of error and c) what is the 95% confidence interv

### Confidence Intervals for a Population Mean

A random sample of 144 observations has a mean of 20, a median of 21 and a mode of 22. The population standard deviation is known to equal 4.8. What is the 95.44% confidence interval for the population mean?

### Finding the Confidence Interval for Mean Number of Hours

5. A researcher wants to estimate the mean number of hours (mu) ; children watch television per day. A sample of 38 children is taken. The sample mean is 6.8 hours with a sample standard deviation of 1.1 hours. a. Find the 90% confidence interval for mean (mu). b. Find the 95% confidence interval for mean (mu).

### Determining the confidence interval for the mean

4. The owner of a business wanted to test how long employees were spending on the internet. A random sample of 49 employees showed a mean of 42 minutes and a standard deviation of 5.2 minutes. Find a 90% confidence for the mean.

### Confidence interval for mean

2. A physician wanted to estimate the mean length of time &#956; that a patient wanted to see him after arriving. A random sample of 64 patients showed a mean waiting time of 23.4 minutes and a standard deviation of 7.1 minutes. Find a 95% confidence interval for &#956;.

### Confidence interval estimation and normal distribution

A laboratory tested eleven chickens and found that the mean amount of cholesterol was 244 milligrams with s = 24.92 milligrams. Construct a 95 percent confidence interval for the true mean cholesterol content of all such eggs. Assume that the population has a normal distribution.

### Confidence interval estimation problem

1. A group of 47 randomly selected students have a mean score of 20.2 with a standard deviation of 4.6 on a placement test. Construct a 90% confidence interval for the mean score, &#61549;, of all students taking the test.

### Confidence intervals to estimate population proportion

Use the given sample data to find the margin of error, E, for the 95% confidence interval used to estimate the population proportion p: n = 186 x= 103

### Confidence Interval

The American Restaurant Association collected information on the number of meals eaten outside the home per week by young married couples. A survey of 60 couples showed the sample mean number of meals eaten outside the home was 2.76 meals per week, with a standard deviation of 0.75 meals per week. Construct a 97 percent confiden

### Confidence Interval

A recent survey of 50 executives who were laid off from their previous position revealed it took a mean of 26 weeks for them to find another position. The standard deviation of the sample was 6.2 weeks. Construct a 95 percent confidence interval for the population mean. Is it reasonable that the population mean is 28 weeks?

### Population mean

A state meat inspector in Iowa has been given the assignment of estimating the mean net weight of packages of ground chuck labeled "3 pounds." Of course, he realizes that the weights cannot be precisely 3 pounds. A sample of 36 packages reveals the mean weight to be 3.01 pounds, with a standard deviation of 0.03 pounds. a. Wh

### Confidence Interval Estimation for Proportion

A local health center noted that in a sample of 400 patients 80 were referred to them by the local hospital. What size sample would be required to estimate the proportion of hospital referrals with a margin of error of 0.04 or less at 95% confidence?

### Margin of error calculation

"In order to estimate the average time spent on the computer terminals per student at a local university, data were collected for a sample of 81 business students over a one week period. Assume the population standard deviation is 1.8 hours. With a 0.95 probability, the margin of error is approximately."

### Confidence Interval

A researcher is interested in determining the average number of years employees of a company stay with the company. If past information shows a standard deviation of 7 months, what size sample should be taken so that at 95% confidence the margin of error will be 2 months or less?"

### Confidence Interval for Time Spent on Computer Terminals

In order to estimate the average time spent on the computer terminals per student at a local university, data were collected for a sample of 81 business students over a one week period. Assume the population standard deviation is 1.8 hours. If the sample mean is 9 hours, then the 95% confidence interval is?

### Hypothesis Testing and Confidence Interval Problems

1. Two teachers give the same standardized statistics test. The results are: Teacher 1 Mean=75 N=20 Teacher 2 Mean=80 N=15 a. What would be a 95% confidence interval for each of the classes? b. The true mean for the test is 70, with a standard deviation of 5. Can you draw any conclusions about either of the clas

### Confidence interval and Normal distributions

In a class there are 60 students. The mean exam score is 70 and the standard deviation is 5. (Be Precise in your answers!) Assume symmetric distribution of the scores. (i) What can you say about the number of students that have scores between 55 and 85? (ii) What can you say about the number of students who ha

### Estimation:

The management of a department store is interested in estimating the difference between the mean credit purchases of customers using the store's credit card versus those customers using a national major credit card. You are given the following information. Store's Card Major Credit Card Sample size 64 49 Sample mean \$140 \$1

### Confidence Interval for Dave the Jogger

Dave the jogger runs the same route every day. On several consecutive days he recorded the number of steps he took using a pedometer, which ranged from 3,103 to 3,450. What sample size would you need to estimate Dave's mean steps with 95 percent confidence and an error of 25 steps? Explain your assumption about .

### Point and Interval Estimates, Confidence Intervals, Levels of Significance and Hypothesis Testing

1. In your own words explain the difference between a point estimate and an interval estimate of a parameter? Which is better? Why? 2. What information is necessary to calculate a confidence interval? Is the size of the population relevant when one is determining the sample size for a confidence interval? What is necessar

### Statistics - Confidence Interval.

The following random sample from a population whose values were normally distributed was collected. 10 8 11 11 The 95% confidence interval for m is a) 8.52 to 10.98 b) 7.75 to 11.75 c) 9.75 to 10.75 d) 8.00 to 10.00

### CONFIDENCE INTERVAL

The 95 percent confidence interval states that 95 percent of the sample means of a specified sample size selected from a population will lie within plus and minus 2.58 standard deviations of the hypothesized population mean. TRUE/FALSE

### Confidence Level Interpretation

For a given confidence interval, what is the interpretation of a 96% confidence level? a. 96% chance that the given interval includes the true value of the population parameter b. Approximately 96 out of 100 such intervals would include the true value of the population parameter c. 4% chance that the given interv

### Confidence Intervals for proportions

The Department of Transportation reported Amtrak trains had a 78% on-time arrival record over the previous 12 months (USA Today, November 23, 1998). Assume that in 2001, a study found 330 of 400 Amtrak trains arrived on time. Does the sample indicate the Amtrak arrival rate has changed? Test at a 95% confidence interval.

### Finding confidence interval for population proportion

Of 900 consumers surveyed, 414 said they were very enthusiastic about a new home decor scheme. What is the 99% confidence interval for the population proportion? a. 30 and 40 b.30 and 60 c.31 and 51 d.42 and 50

### Confidence interval for mean methods

The association of coffee growers in Puerto Rico wants to know the average pounds of coffee consumption per week in towns in the metropolitan area. In a study conducted recently by the average of 81 persons was 1.5 pounds a week and a standard deviation of 0 .10. 1) What is the value of the average population? 2) What distri

### Confidence interval calculations for means

Consider the following random sample of diameter measurements (in inches) of 18 softballs: 4.88, 4.88, 4.76, 4.82, 4.69, 4.75, 4.72, 4.86, 4.81, 4.75, 4.86, 4.73, 4.78, 4.72, 4.73, 4.78, 4.71, 4.87 If we assume that the diameter measurements are normally distributed, find a confidence interval for the mean diameter of

### Frequcny distribution chart and confidence interval

Scenario: You are an associate assigned to the claims department of a major insurance company. A policy holder has had an accident with his classic 1968 Oldsmobile Cutlass Supreme. At issue is the consideration of his brakes. Recently, he was driving down the road and was apparently unable to stop in time when a woman driv