Question

Suppose you have a sample size of 50 with a mean 77 and a population standard deviation of 8.3. Based on this, construct a 95% confidence interval for the true population mean. Use z = 2 (rather than 1.96) for your calculations. Give your answers as decimals, to two places

In a survey, 19 people were asked how much they spent on their child's last birthday gift. The results were roughly shaped as a normal curve with a mean of $46 and standard deviation of $9. Find the margin of error at a 90% confidence level.
Give your answer to two decimal places.

In a survey, 15 people were asked how much they spent on their child's last birthday gift. The results were roughly shaped as a normal curve with a mean of $45 and standard deviation of $5. Construct a confidence interval at a 80% confidence level.
Give your answers to one decimal place.

±±

Answer 1

1.

z = 2

95% confidence interval for the true population mean

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2.

At = 1-0.90 = 0.10 and df = 19-1=18, critical value, = T.INV.2T(0.10, 18) = 1.734

Margin of error, E =

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3.

At = 1-0.80 = 0.20 and df = 15-1=14, critical value, = T.INV.2T(0.20, 14) = 1.345

80% confidence interval: