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# Confidence Interval

### Confidence Interval for Population Proportion using Excel

Question: In a survey of 842 musicians, 715 said they began playing an instrument while in elementary school. Using Excel, construct a 90% confidence interval for the proportion of musicians who began playing an instrument while in elementary school.

### Confidence Interval Problem: Sample Size for Sales Call Logs

Question: A company wants to estimate the average number of sales calls received daily. They want to be within 10 of the true mean when using a confidence interval of 95%. The standard deviation is estimated at 20. How many daily call logs must they sample?

### Standard Error and 95% Confidence Interval Calculations

The State of Michigan tested 766 water samples from private wells in Lapeer County, Michigan, and found that 112 exceeded the EPA guidelines of 50 ppb. (a) Find the standard error of the estimated proportion. (b) Find the 95 percent confidence interval width for the true proportion. (c) Is normality an issue here? Expla

### Statistics: Confidence Interval Problem

See the attached Q8 file for proper format. Only use the attached documents to assist with your solution and a scientific calculator. Please show all working out so I can understand how you have solved the question, and do not use Excel. Q8 A study was carried out by JT Co. Regarding the stopping distance of a 1300 kilogr

### Confidence intervals for employees who took work on vacation

In a recent survey, 161 of 493 employees responded that they typically took work with them on vacation and 95 responded that there are unwritten and unspoken expectations that they stay connected. a. Construct a 90% confidence interval estimate for the population proportion of employees who typically take work with them on va

### Confidence interval for the population mean total fat

The data shown below represents the total fat, in grams per serving, for a sample of 20 chicken sandwiches from fast-food chains. Complete parts (a) through (d). 5 4 6 6 19 17 24 27 20 33 18 28 25 17 29 27 35 26 38 63 a. Construct a 99% confidence interval for t

### Confidence Interval

Can you show how to do this in Excel if possible? Question: 1. A scientist wants to estimate the proportion of plants with berries growing in a particular region. They want to be within 6% of the true proportion when using a confidence interval of 99%. How many plants must be sampled if no preliminary estimate is avai

### Confidence & Prediction Intervals for Light Bulb Life

An electrical firm manufactures light bulbs that have a length of life that is approximately normally distributed. If a sample of 16 bulbs has an average life of 50 hours and the population standard deviation is known to be 10 hours. a) What is the 94% confidence interval for the population mean of all bulbs produced by the

### T-based Confidence Interval for Population Mean

The GPAs of 15 randomly sampled students are shown in the table below: (See Attachment) Using Excel, construct a 95% confidence interval using a t-distribution for the mean GPA. 95% confidence interval: (_____+/-_____) Please show me how this is accomplished in Excel.

### Finding a Confidence Interval for a Population Mean

1. Given the 95% confidence interval (75.23 +/- 3.16) for a population mean, rewrite in the following alternative format with left and right endponts: ( , ) 2. Would a given mean value of 79.56 be likely, given the calculated confidence interval?

### Confidence Interval for mean university students

A University surveys 36 randomly selected students and asks the number of dollars sent on textbooks/supplies during the academic year. The results (in dollars) are shown below: [See Attachment] Use Excel to construct a 90% confidence interval for the mean dollars spent. Would a given mean value of \$600 be likely, given

### Testing of Hypothesis and Confidence Interval of Population Mean

Temperature measurements are recorded at 40 randomly selected different locations for a particular city. A sample mean of 20 degrees Celsius is obtained with a standard deviation of 1.5 degrees.. There are no outliers in the sample measurements. Test the claim that the population mean is 21 degrees Celsius. Use a 0.05 significan

### Confidence Intervals for mean; t test; hypothesis test for proportion

Confidence intervals/One sample hypothesis tests Directions: For #1, calculate and interpret the confidence interval. Please show your manual calculation or the software output. Please show your work if you calculated manually on #1. If you used statistical software, please show the output. 1: Confidence Intervals for t

### Amazon.com claims that its textbook prices are cheaper than those offered by the local college bookstore. To test this claim, you randomly select 15 textbooks that are sold by both. Data File TEXTBOOK gives the prices for those 15 randomly selected textbooks at the local college bookstore and at Amazon.com.

6. Amazon.com claims that its textbook prices are cheaper than those offered by the local college bookstore. To test this claim, you randomly select 15 textbooks that are sold by both. Data File TEXTBOOK gives the prices for those 15 randomly selected textbooks at the local college bookstore and at Amazon.com. a. Construct

### A market researcher hired by Pepsi-Cola: Confidence interval for Pepsi preference

See attached data file. A market research consultant hired by the Pepsi-Cola Co. is interested in determining the proportion of consumers who favor Pepsi-Cola over Coke Classic in a particular urban location. A random sample of 250 consumers from the market under investigation is provided in P09_17.xlsx. Construct a 90% c

### Tests from confidence intervals

A confidence interval for the population mean tells us which values of mean are plausible (those inside the interval) and which values are not plausible (those outside the interval) at the chosen level of confidence. You can use this idea to carry out a test of any null hypothesis. Ho:mean =Mo starting with a confidence interva

### Margin of Error and Confidence Interval for a Poll

A recent poll (fictitious) reports that President Obama's "approval rating" is 52% and a statement says, "This poll has an error rate of 3%." The poll sampled 1300 people. (Note: The "3%" is the "margin of error." There is no computation of the confidence interval formula needed). This is an application of a confidence inter

### Confidence level vs Type 1 error; hypotheses for music lessons

A) What is the relationship between the Confidence Level and Type I Error? b) State the null and alternative hypotheses for the following: You are trying to show that music lessons have a positive effect on a child's self-esteem.

### AluminiCorp: Test Random Sample and Confidence Interval

AluminiCorp is a major producer of aluminum cans that produces 40 billion aluminum cans every year. You work as a quality control officer for AluminiCorp, and are responsible for ensuring that the aluminum cans produced meet certain specifications. Each can is supposed to consist of precisely 15 grams of aluminum, and the market

### Repeated weighings of a specimen for a 95% confidence interval

A laboratory scale is known to have a standard deviation of = 0.001 gram in repeated weighings. Scale readings in repeated weighings are normally distributed, with mean equal to the true weight of the specimen. How many times must you weigh a specimen on this scale in order to get a margin of error no larger than +0.0005 (+ ha

### Confidence Interval: Mean load needed to pull apart pieces of Douglas fir

How heavy a load (pounds) is needed to pull apart pieces of Douglas fir 4 in long and 1.5 inches square? Here are the data from students doing a laboratory exercise: 33,190 31,300 32,320 26,520 23,040 30,460 24,050 33,650 31,860 28,730 33,020 33,280 30,930 32,700 30,170

### Confidence Interval Validity for the Study Time of Students

A class survey in a large class for first year college students asked, "About how many minutes do you study on a typical weeknight?" The mean response of the 269 students was x (bar over the x) = 137 minutes. Suppose that we know that the study time follows a normal distribution with standard deviation = 65 minutes in the popula

### Statistics: Sample variability, and Introduction to statistical inferences

1. Consider a normal population with µ = 25 and &#963; = 8.0. (A) Calculate the standard score for a value x of 27. (B) Calculate the standard score for a randomly selected sample of 30 with = 27. (C) Explain why the standard scores of 27 are different between A and B above 2. Assume that the mean score on

### Calculate sample size, maximum error of estimate

Assume that a sample is drawn and z(?/2) = 1.96 and Ï? = 15. Answer the following questions: (A) If the Maximum Error of Estimate is 0.04 for this sample, what would be the sample size? (B) Given that the sample Size is 400 with this same z(?/2) and Ï?, what would be the Maximum Error of Estimate? (C) Wha

### Finding Confidence Intervals: Golf Example

A sample of 140 golfers showed that their average score on a particular golf course was 93.38 with a standard deviation of 4.36. Answer each of the following (show all work and state the final answer to at least two decimal places): (A) Find the 90% confidence interval of the mean score for all 140 golfers. (B) Find the

### A mayoral election probability

A mayoral election race is tightly contested. In a random sample of 1,100 likely voters, 572 said they were planning to vote for the current mayor. Based on this sample, what is your initial hunch? If you calculated the 95% confidence interval around the mayorâ??s current proportion of votes, would you claim with 95% confidence

### Calculate price/value for share; standard deviation of security; rental agreement choice

9.4 You own one share in a company called Invest Co. Inc. Examining the balance sheet, you have determined that the firm has \$100,000 cash, equipment worth \$900,000, and 100,000 shares outstanding. Calculate the price/value of each share in the firm, and explain how your wealth is affected if: A. The firm pays out dividen

### Confidence Intervals: Z-Tests

Digital Switches, Inc., produces a product called the C3PO with an advertised 0.5% failure rate. A test of products shipped from the factory to the company's testing lab found a sample average failure rate of 0.45% with a sample standard deviation of 0.15% when a sample of n = 64 observations was studied. A. Calculate the ra

### Statistics: 35 golfers' scores, price of pizza, weight of wrestlers, business majors

1. A study of 35 golfers showed that their average score on a particular course was 92. The standard deviation of the population is 5. A. Find the best point estimate of the mean. B. Find the 95% confidence interval of the mean score for all golfers. C. Find the 95% confidence interval of the mean score if a sample of 60 golf

### Random Sampling in EXCEL and Confidence Interval Estimate

The employee benefits manager of a small private university would like to estimate the proportion of full-time employees who prefer adopting the first (i.e., plan A) of three available health care plans in the coming annual enrollment period. A reliable frame of the university's employees and their tentative preferences are in t