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CONFIDENCE INTERVALS
Feb. 18th, 2015
A STATS PROFESSOR ASKED HER STUDENTS WHETHER OR NOT THEY WERE REGISTERED
TO VOTE. IN A SAMPLE OF 50 OF HER STUDENTS (RANDOMLY SELECTED), 35 OF
THEM SAID THEY WERE REGISTERED TO VOTE.
1. Find a 95% confidence interval for the true proportion of the professor’s students who were registered to vote. (Just find the interval; no conclusion)
A STATS PROFESSOR ASKED HER STUDENTS WHETHER OR NOT THEY WERE REGISTERED
TO VOTE. IN A SAMPLE OF 50 OF HER STUDENTS (RANDOMLY SELECTED), 35 OF
THEM SAID THEY WERE REGISTERED TO VOTE.
2. Now, interpret the confidence interval in this context (conclusion).
A STATS PROFESSOR ASKED HER STUDENTS WHETHER OR NOT THEY WERE REGISTERED
TO VOTE. IN A SAMPLE OF 50 OF HER STUDENTS (RANDOMLY SELECTED), 35 OF
THEM SAID THEY WERE REGISTERED TO VOTE.
3. According to a September 2004 Gallup poll, about 73% of 18 to 29 year olds said they were registered to vote. Does this 73% figure from Gallup seem reasonable for the professor’s students? Explain.
A STATS PROFESSOR ASKED HER STUDENTS WHETHER OR NOT THEY WERE REGISTERED
TO VOTE. IN A SAMPLE OF 50 OF HER STUDENTS (RANDOMLY SELECTED), 35 OF
THEM SAID THEY WERE REGISTERED TO VOTE.
4. If the professor only knew the information from the September 2004 Gallup poll and wanted to estimate the percentage of her students who were registered to vote to within ±4% with 95% confidence, how many students should she sample?
ON JANUARY 30TH, 2007, FOX NEWS POLLED 900 REGISTERED VOTERS
NATIONWIDE. WHEN ASKED, “DO YOU BELIEVE GLOBAL WARMING EXISTS?” 82%
SAID “YES”.
1. Find a 90% confidence interval for the true proportion of Americans who believe global warming exists. (Just find the interval; no conclusion)
ON JANUARY 30TH, 2007, FOX NEWS POLLED 900 REGISTERED VOTERS
NATIONWIDE. WHEN ASKED, “DO YOU BELIEVE GLOBAL WARMING EXISTS?” 82%
SAID “YES”.
2. Now, interpret the confidence interval in this context (conclusion).
ON JANUARY 30TH, 2007, FOX NEWS POLLED 900 REGISTERED VOTERS
NATIONWIDE. WHEN ASKED, “DO YOU BELIEVE GLOBAL WARMING EXISTS?” 82%
SAID “YES”.
3. Suppose an environmental group planning a follow-up survey of voters’ opinions on global warming wants to determine a 95% confidence interval with a margin of error of no more than ±2%. How large a sample do they need?
