A study of atmospheric pollution on the slopes of Blue Ridge Mountains (Tennessee) was conducted. The file leadmoss.txt haves the levels of lead found in 70 fern moss specimens (in micrograms of lead per gram of moss tissue) gathered from the mountain slopes, as well as the elevation of moss specimen (in feet) and the direction (1 if east, 0 if west) of the slope face.
a. Write down the equation of first-order model relating mean lead level, E(y), to elevation (x1) and clope face (x2). Include interaction between elevation and slope face in model.
b. Graph the relationship between mean lead level and elevation for different slope faces that is hypothesized by model in part a.
c. In terms of the β's of the model, part a, provide the change in lead level for every one foot raise in elevation for moss specimens on the east slope.
d. Fit the model, part a, to the data using an available statistical software package. Is the entire model statistically useful for predicting lead level? Test using α = 0.10.