The article "Distributions of Compressive Strength Obtained from Various Diameter Cores" (ACI
Materials J., 2012: 597-606) described a study in which compressive strengths were determined for concrete specimens of various types, core diameters, and length to-diameter ratios. For one particular type, diameter, and l/d ratio, the 18 tested specimens resulted in a sample mean compressive strength of 64.41 MPa and a sample standard deviation of 10.32 MPa. Normality of the compressive strength distribution was judged to be quite plausible.
a. Calculate a confidence interval with confidence level 98% for the true average compressive strength under these circumstances.
b. Calculate a 98% lower prediction bound for the compressive strength of a single future specimen tested under the given circumstances.