Coir fiber, derived from coconut, is an eco-friendly material with great potential for use in construction. The article

“Seepage Velocity and Piping Resistance of Coir Fiber Mixed Soils” (J. of Irrig. and Drainage Engr., 2008:

485–492) included several multiple regression analyses.

The article’s authors kindly provided the accompanying data on

, and seep

b. Fitting the model with predictors x3, x4, and the interaction x3x4 gave . Does this model appear to be useful? Can an F test be used to compare this model to the model of (a)? Explain.

c. Fitting the model with predictors as well as all second-order interactions gave (this model was also fit by the investigators). Does it appear that at least one of the interaction predictors provides useful information about power over and above what is provided by the first-order predictors? State and test the appropriate hypotheses using a significance level of .05.