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# Confidence Interval of Test Score Sample

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1. The standard deviation of test scores on a certain achievement test is 10.7. A random sample of 60 scores on this test had a mean of 76.2. Based on this sample, find a 90% confidence interval for the true mean of all scores.

What is the lower limit of 90% confidence interval?
What is the upper limit of the 90% confidence interval?

Carry your intermediate computations to at least three decimal places. Round your answers to one decimal place.

2. Many college graduates who are employed full-time have longer than 40-hour work weeks. Suppose that we wish to estimate the mean number of hours, worked per week by college graduates employed full-time. We'll choose a random sample of college graduates employed full-time and use the mean of this sample to estimate . Assuming that the standard deviation of the number of hours worked by college graduates is 6.10 hours per week, what is the minimum sample size needed in order for us to be confident that our estimate is within 1.4 hours per week of ?

Carry your intermediate computations to at least three decimal places. Write your answer as a whole number (and make sure that it is the minimum whole number that satisfies the requirements).

https://brainmass.com/statistics/confidence-interval/confidence-interval-test-score-sample-362466

#### Solution Preview

1. The standard deviation of test scores on a certain achievement test is 10.7. A random sample of 60 scores on this test had a mean of 76.2. Based on this sample, find a 90% confidence interval for the true mean of all scores. Then answer:

What is the lower limit of 90% confidence interval?
What is the upper limit of the 90% confidence interval?

Carry your intermediate computations to at least three decimal places. Round your answers to one decimal place.