question archive The accompanying table gives summary data on cube compressive strength (N/mm2) for concrete specimens made with a pulverized fuel-ash mix
The accompanying table gives summary data on cube compressive strength (N/mm2) for concrete specimens made with a pulverized fuel-ash mix
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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 67 27.98 4.85 28 75 36.74 6.41 Calculate a 99% CI for the difference between true average 7-day strength and true average 28-day strength. (Use H0: μ7 − μ28 = 0. Round your answers to two decimal places.) ,
( , )N/mm2
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Answer:
Age (days) Sample Size Sample Mean Sample SD
7 67 27.98 4.85
28 75 36.74 6.41
(μ7 − μ28 )=27.98-36.74=-8.76
Standard error difference of average (μ7 − μ28 )=sqrt( 12/n1+
22/n2) =sqrt(4.85*4.85/67 +6.41*6.41/75)=0.9481
(1-alpha)*100% confidence interval for (μ7 − μ28 )=(μ7 − μ28 )±z(alpha/2)*SE(μ7 − μ28 )
95% confidence interval for (μ7 − μ28 )=-8.76±z(0.05/2)*0.9481=-8.76±1.96*0.9481=-8.76±1.86=(-10.62,-6.9)
