Construct a confidence interval at a 80% confidence level. Give your answers to one decimal place.

Question

In a survey, 18 people were asked how much they spent on their child's last birthday gift. The results were roughly bell-shaped with a mean of $31 and standard deviation of $2. Construct a confidence interval at a 80% confidence level.

Answer 1

CI = mean +/- critical t-value * SD/sqrt(sample size)

mean = 31

SD = 2

sample size = 18

critical t value for 80% confidence level and sample size 18 = t-value for 10% tail probability and 18-1 = 17 degrees of freedom = 1.333.

CI = 31 +/- 1.333 * 2/sqrt(18) = (30.37,31,63)