In a survey of 250 voters prior to an election 40 indicated

In a survey of 250 voters prior to an election, 40% indicated that they would vote for the incumbent candidate. Construct a 95% confidence interval for the population proportion of voters who support the incumbent.

Solution

alpha = 1 - 0.95 = 0.05

alpha/ 2 = 0.025

Z = 1.96

p = 0.40

I : p +/- Z *srqt ( p*q / n )

I : 0.40 +/- 1.96 * srqt [(0.40*0.60) / 250]

I: 0.40 +/- 0.0607

I : [ 0.3393 , 0.4607 ]