7. A study of the properties of metal plate—connected trusses used for roof support ( Modeling Joints Made with Light-Gauge Metal Connector Plates, Forest Products J., 1979: 39—44) yielded the following observations on axial stiffness index (kips/in.)

for plate lengths 4, 6, 8, 10, and 12 in.:

4: 309.2 409.5 311.0 326.5 316.8 349.8 309.7 6: 402.1 347.2 361.0 404.5 331.0 348.9 381.7 8: 392.4 366.2 351.0 357.1 409.9 367.3 382.0 10: 346.7 452.9 461.4 433.1 410.6 384.2 362.6 12: 407.4 441.8 419.9 410.7 473.4 441.2 465.8

a. Check the ANOVA assumptions with a normal plot and a test for equal variances.

b. Does variation in plate length have any effect on true average axial stiffness? State and test the relevant hypotheses using analysis of variance with a  .01. Display your results in an ANOVA table. (Hint:  5,241,420.79.)