A researcher wanted to build up a model to predict the yearly population size of certain species of mice in a mountainous area based on a set of environmental variables, including human population density, coverage of bamboo (a major food source for mice in the area), annual rain precipitation, and length of summer (the breeding period for mice in the area). She had the data from the past n = 30 years. For the i year, the mouse population size (y_i , in thousands), human population density (x_i1 ), bamboo coverage (x_i2 ), rain precipitation (x_i3 ), and length of summer (x_i4 , in days) were recorded.

The researcher first fit a multiple linear regression with the response mouse population size (y ) and the four predictor variables x , x , x , and x . Below is the Externally Studentized Residuals vs. Predicted plot.

1 Externally Studentized Residuals -2 -50 50 100 Predicted What does the horizontal axis represent in this residual plot? O Predicted predictor value at each observation O Predicted response value at each observation O Observed response value at each observation O Observed predictor value at each observation