Complete question :
In March 2003, the Pew Research Group surveyed 1508 adult Americans and asked, "Do you believe the United States made the right or wrong decision to use military force in Iraq?" Of the 1508 adult Americans surveyed, 1086 stated the United States made the right decision. In August 2010, the Pew Research Group asked the same question of 1508 adult Americans and found that 618 believed the United States made the right decision. Construct and interpret a 90% confidence interval for the difference between the two population proportions
Answer:
(0.282 ; 0.338)
We are 90% confident that the proportion of adults in the U.S who believe that the United States made the right decision in 2003 lies between 0.282 and 0.338, higher than the proportion of adult Americans in 2010 who believe the same.
Explanation:
Given :
p = x / n
p1 = 1086/1508 = 0.72
p2 = 618/1508 = 0.41
Confidence interval :
(p1 - p2) ± margin of error
Margin of Error = Zcrit*√p1(1-p1)/n1 + p2(1-p2)/n2
Zcrit at 90% = 1.645
1 - p1 = 1 - 0.72 = 0.28
1 - p2 = 1 - 0.41 = 0.59
MOE = 1.645*√(0.72*0.28)/1508 + (0.41*0.59)/1508
MOE = 1.645 * 0.0171492 = 0.0282105
(p1 - p2) = (0.72 - 0.41) = 0.31
Lower boundary = 0.31 - 0.0282 = 0.2818
Upper boundary = 0.31 + 0.0282 = 0.3382
(0.282 ; 0.338)
We are 90% confident that the proportion of adults in the U.S who believe that the United States made the right decision in 2003 lies between 0.282 and 0.338, higher than the proportion of adult Americans in 2010 who believe the same.