The accompanying table gives summary data on cube compressive strength (N/mm^2)

Question

The accompanying table gives summary data on cube compressive strength (N/mm^2) for concrete specimens made with a pulverized fuel-ash mix. Calculate a 99% CI for the difference between true average 7-day strength and true average 28-day strength. (Use H_0: mu_7 - mu_28 = 0. Round your answers to two decimal places.)() N/mm^2 Interpret your 99% CI With 99% confidence, that the true difference falls outside these values. With 99% confidence, that the true difference falls below the lower bound. With 99% confidence, that the true difference falls above the upper bound. With 99% confidence, we can say that the true difference falls between these values.

Explanation / Answer

z value at 99% CI = 2.576

CI = ( x1 - x2) + / - z * sqrt ( s1^2 / n1 + s2^2/n2)

= ( 25.96 - 36.72) +/- 2.576 * sqrt ( 4.82^2 / 65 + 6.48^2 / 73)
= (-13.247 , -8.272)

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