Before
33
33
38
33
35
35
40
40
40
31
After
34
28
25
28
35
33
31
28
35
33
Ten different families are tested for the number of gallons of water per day they use before and after viewing a conservation video. Construct a 90% confidence interval for the mean of the differences.
Subtract before values from after values.
Find the mean = sum of scores/number of scores
Subtract each of the scores from the mean and square each difference. Find the sum of these squares. Divide that by the number of scores to get variance.
Standard deviation = square root of variance
90% = mean ± 1.645 SD
I'll let you do the calculations.