# Example 8.9, Determinants of Personal Computer Ownership
# Data set: gpa1

# Function for result reporting
source("_report.R")

# Load the data, create new variable and estimate the model
load("gpa1.Rdata")
data$parcoll=as.numeric(!(data$fathcoll==0&data$mothcoll==0))
model1=lm(PC~hsGPA+ACT+parcoll,data=data)
dig1=c(4,3,4,3,4)

# Describe the model
cat("Model to estimate: PC = beta0 + beta1 * hsGPA + beta2 * ACT + beta3 * parcoll + u",
    "\nwhere PC is ", paste(desc[desc[,1]=="PC",2]),
    "\nhsGPA is ", paste(desc[desc[,1]=="hsGPA",2]),
    "\nACT is ", paste(desc[desc[,1]=="ACT",2]),
    "\nand parcoll is =1 if at least one parent attended college",
    sep="")

# Report results
{
  cat("The estimated regression line is")
  reportreg(model1,dig1,HC=T)
}

# Estimate by WLS
data$fitted=model1$fitted.values
model2=lm(PC~hsGPA+ACT+parcoll,weight=1/(fitted*(1-fitted)),data=data)
dig2=c(3,3,4,3,4)
{
  cat("There is little difference between both sets of standard errors. Nevertheless, we estimate the model again by WLS. Because all of the OLS fitted values are inside the unit interval, no adjustments are needed. The estimated regression line is")
  reportreg(model2,dig2)
}

# Interpretation
cat("There are no important differences in the OLS and WLS estimates. The only significant explanatory variable is parcoll, and the magnitudes of its coefficients in both estimates are close")