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Cell phone SAR levels have a known population standard deviation of σ = 0.337. A sample of 30 phones gives x-bar = 1.024. For a 98% confidence level, z(0.01) = 2.326. What is the resulting confidence interval?
A(0.6870, 1.3610)
B(0.8809, 1.1671)
C(1.0240, 1.1671)
D(0.8809, 1.0240)
Answer & Solution
Correct answer: B. (0.8809, 1.1671)
1. EBM = z(σ/sqrt(n)) = (2.326)(0.337/sqrt(30)) ≈ (2.326)(0.0615) ≈ 0.1431.
2. Lower bound: 1.024 - 0.1431 ≈ 0.8809.
3. Upper bound: 1.024 + 0.1431 ≈ 1.1671.
4. Options C and D each apply the error bound to only one side of x-bar, producing an interval that is not centered on the point estimate.
_Source: OpenStax Introductory Statistics (CC BY 4.0), Ch 8 "Confidence Intervals", section 8.1 A Single Population Mean using the Normal Distribution_
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