In a study of pulverized fuel-ash concrete mix, it was found that the sample mean compressive...

In a study of pulverized fuel-ash concrete mix, it was found that the sample mean compressive strength was 27.0 with a standard deviation of 4.89 for a sample of 68 specimens which had been aged for 7 days. A sample of 74 specimens which had been aged for 28 days yielded a mean compressive strength of 35.8 with standard deviation 6.43. Is there strong evidence that mean compressive strength is higher after 28 days than after 7 days? Yes, because an approximate 95% confidence interval for the difference in mean compressive strength is 8.8 ± 1.9. No, because an approximate 95% confidence interval for the difference in mean compressive strength is 8.8 ± 1.9. Yes, because an approximate 95% confidence interval for the difference in mean compressive strength is 8.8 ± 3.1. No, because an approximate 95% confidence interval for the difference in mean compressive strength is 8.8 ± 3.1. No, because a paired analysis is required for this kind of data:

Yes, because an approximate 95% confidence interval for the difference in mean compressive strength is 8.8 ± 1.9

No, because an approximate 95% confidence interval for the difference in mean compressive strength is 8.8 ± 3.1. Yes, because an approximate 95% confidence interval for the difference in mean compressive strength is 8.8 ± 3.1.

No, because an approximate 95% confidence interval for the difference in mean compressive strength is 8.8 ± 1.9.

No, because a paired analysis is required for this kind of data.

Solutions

Expert Solution

x₁ =	35.80	x₂ =	27.00
n₁ =	74	n₂ =	68
σ₁ =	6.43	σ₂ =	4.89

std error σ_x_1-_x₂=√(σ²1/n₁+σ²₂/n₂) =			0.954

Point estimate of differnce '=x1-x2 =			8.800
for 95 % CI value of z=			1.960
margin of error E=z*std error =			1.9

correct option is:

Yes, because an approximate 95% confidence interval for the difference in mean compressive strength is 8.8 ± 1.9

(since interval values are above 0)

orchestra answered 2 years ago

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