1. The quality control manager at a light bulb factory needs to estimate the mea

Question

1. The quality control manager at a light bulb factory needs to estimate the mean life of a large shipment of light bulbs. The population standard deviation is known to be 100 hours. A random sample of 36 light bulbs indicated a sample mean life of 350 hours.

a. What is the critical t value for a 99% confidence interval using a sample size of 44? Use Excel instead of the table in your text to get a precise answer.

2. A stationery store wants to estimate the mean retail value of greeting cards that it has in its inventory. A random sample of 35 greeting cards indicates an average value of $1.67 and a sample standard deviation of $0.32. Set up a 95% confidence interval estimate of the mean value of all greeting cards in the store’s inventory. (Assume the population is normally distributed.)

[HINT: Think about which confidence interval formula you want to use here.]

[Show your work.]

3. Given a population with a mean of 70 and a standard deviation of 15, which of the following is most unusual?

An individual with a score of 85?

An individual with a score of 60?

A sample of n = 25 with a mean of 65?

Explain your answer.

Explanation / Answer

Mean = 70

Stdev = 15

Unusual is anything which is more than Z=2 or -2 away from mean

So, 85 = 70+15 ( 1 deviation away) - not an outlier

60 = 70-10 ( within 1 deviation) = - not an outlier

65 = (65-70) / (15/sqrt(25)) = -1.67 within limits = not an outlier

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