Confidence Intervals

Question

Out of 400 people sampled, 176 preferred Candidate A. Based on this, estimate what proportion of the voting population () prefers Candidate A.

Use a 95% confidence level, and give your answers as decimals, to three places.

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Answer 1

CI = sample proportion (phat) +/- z-critical * sqrt(phat*(1-phat)/sample size))

phat = 176/400, the proportion that favored candidate A

sample size = 400

z-critical for 95%: CI is middle 95% of area under normal curve and thus there is 2.5% on each end, so z-critical is probability from standard normal probability table associated with 97.5%.