Can you help me w/ a Confidence interval statistics problem?

Question

If you can help me out here..i'd APPRECIATE IT GREATLY!!!!!!!!

SRS of 625 ppl...a 99% Conf Interv gives us 126.45 - 128.55. Someone who's above 140 is "at-risk". Can we calculate a 90% for proportion of "at-risk" individuals??? If you can...what is it?

THAAANKS!! *muah*

Answer 1

Since 140 is not a random variable you can't form a confidence interval surrounding it. The only inference you can make is the probability of someone from the same population exceeding 140.

If you want to calculate the 90th percentile of those over 140 you can use Baye's rule stating of P(A|B)=P(AnB)/P(B). In this case you want P(X>x)/P(x>140)=.9 and employing normalcy.

Since 140 is over 30 standard deviations over the mean you can assume the probability of x being over 140 to be zero.

Hope that helps.