Lead levels in mountain moss. A study of the atmospheric pollution on the slopes of the Blue Ridge Mountains (in Tennessee) was conducted. The file MOSS contains the levels of lead found in 70 fern moss specimens (in micrograms of lead per gram of moss tissue) collected from the mountain slopes, as well as the elevation of the moss specimen (in feet) and the direction (1 if east, 0 if west) of the slope face. The first five and last five observations of the data set are listed in the following table.

Specimen Lead Level Elevation Slope Face 1 3.475 2,000 0 2 3.359 2,000 0 3 3.877 2,000 0 4 4.000 2,500 0 5 3.618 2,500 0 f f f f 66 5.413 2,500 1 67 7.181 2,500 1 68 6.589 2,500 1 69 6.182 2,000 1 70 3.706 2,000 1

a. Write the equation of a first-order model relating mean lead level E(y) to elevation 1x12 and slope face 1x22. Include interaction between elevation and slope face in the model.

b. Graph the relationship between mean lead level and elevation for the different slope faces that is hypothesized by the model you wrote in part a.

c. In terms of the b’s of the model from part

a, give the change in lead level for every 1-foot increase in elevation for moss specimens on the east slope.

d. Fit the model from part a to the data, using an available statistical software package. Is the overall model statistically useful in predicting lead level? Test, using a = .10.