Lead in fern moss. A study of the atmospheric pollution on the slopes of the Blue Ridge Mountains (Tennessee) was conducted. The file LEADMOSS 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 table.

(a)    Write the equation of a first-order model relating mean lead level, E(y), to elevation (x1) and slope face (x2). Include interaction between elevation and slope face in the model.

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(b) Graph the relationship between mean lead level and elevation for the different slope faces that is hypothesized by the model, part a.

(c) In terms of the β’s of the model, part a, give the change in lead level for every one foot increase 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 overall model statistically useful for predicting lead level? Test using α = .10.

(e) Write the equation of the complete secondorder model relating mean lead level, E(y), to elevation (x1) and slope face (x2).


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