Random samples of students at 112 four year colleges were interviewed several times since 1993 Of the students who reported being sexually active the percentage who reported susing condoms was 49.7% of 12629 students in 1993 and 43.6% of 9756 students in 2004

a Estimate the difference between the proportions in 2004 and 1993 and interpret

b Find the standard error for this difference

c Construct and interpret a 99 confidence interval to estimate the true change

d State the assumptions for the confidence interval in c to be valid