8-15. A civil engineer is analyzing the compressive strength of concrete. Compressive strength is normally distributed with $sigma^2 = 1000(psi)^2$. A random sample of 12 specimens has a mean compressive strength of $ar{x} = 3250$ psi. (a) Construct a 95% two-sided confidence interval on mean compressive strength. (b) Construct a 99% two-sided confidence interval on mean compressive strength. Compare the width of this confidence interval with the width of the one found in part (a).
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A civil engineer is analyzing the compressive strength of concrete. Compressive strength is normally distributed with $\sigma^{2}=1000(\mathrm{psi})^{2}$. A random sample of 12 specimens has a mean compressive strength of $\bar{x}=3250$ psi. (a) Construct a $95 \%$ two-sided confidence interval on mean compressive strength. (b) Construct a $99 \%$ two-sided confidence interval on mean compressive strength. Compare the width of this confidence interval with the width of the one found in part (a).
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A civil engineer is analyzing the compressive strength of concrete. Compressive strength is normally distributed with $\sigma^{2}=1000(\mathrm{psi})^{2} .$ A random sample of 12 specimens has a mean compressive strength of $\bar{x}=3250$ psi. (a) Construct a $95 \%$ two-sided confidence interval on mean compressive strength. (b) Construct a $99 \%$ two-sided confidence interval on mean compressive strength. Compare the width of this confidence interval with the width of the one found in part (a).
Qs. 6 A civil engineer is analyzing the compressive strength of concrete. Compressive strength is normally distributed with σ² = 1000 (psi)². A random sample of 12 specimens has a mean compressive strength of ̄x = 3250 psi. (a) Construct a 99% two-sided confidence interval on mean compressive strength. (b) Construct a 95% two-sided confidence interval on mean compressive strength. Compare the width of this confidence interval with the width of the one found in part (a). (c) Suppose that it is desired to estimate the compressive strength with an error that is less than 15 psi at 95% confidence. What sample size is required?
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