Question

2. An article reports a comparison of several methods for predicting the shear strength for steel plate girders. Data for two of these methods, the Karlsruhe and Lehigh procedures, when applied to six specific girders, are shown in the following table. We wish to determine whether there is any difference (on the average) between the two methods. Use the paired t-test.

Girder	Karlsruhe	Lehigh
S1/1	1.186	1.061
S2/1	1.151	0.992
S3/1	1.322	1.063
S4/1	1.339	1.062
S5/1	1.2	1.065
S2/1	1.402	1.178
S2/2	1.365	1.037

2.1. (2 points) Does each of the individual shear strengths have to be normally distributed for the paired t-test to appropriate, or is it only the difference in shear strengths that must be normal? Use a normal probability plot to investigate the normality assumption. Draw the normal probability plot for the difference using Microsoft Excel Data Analysis Toolpak. What is the R-squared?

2.2. (2 points) Do the data suggest that the two methods provide the same mean shear strength? Use α = 0.01. Find the P-value.

2.3. (2 points) Construct a 99% confidence interval and comment on why the methods are the same or different in mean shear strength.

Answer 1