Question

The accompanying table gives summary data on cube compressive strength (N/mm2) for concrete specimens made with a pulverized fuel-ash mix. Age (days) Sample Size Sample Mean Sample SD 7 4.88 68 73 27.95 34.73 28 6.45 Calculate a 99% CI for the difference between true average 7-day strength and true average 28-day strength. (Use Ho: 47 - 428 = 0. Round your answers to two decimal places.) -16.41 x 2.85 N/mm2 Interpret your 99% CI. With 99% confidence, that the true difference falls below the lower bound. O With 99% confidence, that the true difference falls above the upper bound. With 99% confidence, that the true difference falls outside these values. With 99% confidence, we can say that the true difference falls between these values. You may need to use the appropriate table in the Appendix of Tables to answer this question.

Answer 1

solution:-

by the given above information

degree of freedom df = (n1+n2)-1 = (68+73)-2 = 139

we look into t table with df and with 99% confidence level

critical value t = 2.612

confidence interval formula

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

=> (27.95 - 34.73) +/- 2.612 * sqrt((4.88^2/68) + (6.45^2/73))

=> (-9.29 , -4.27)

interpret your 99% CI

with 99% confidence,we can say that the true difference falls between these values