Homework 4.

0. Look at the Cedegren's Panamanian Spanish Varbrul instructions
 handout to understand the problem and the coding of the data that this
 homework deals with.  You aren't expected to actually do this homework
 (i.e., to actually download and use Varbrul).  Also, download the data,
 so you can load it into R.

1.  Build two logistic regression models from the Cedergren data, one
 using only POS as an independent variable and the other using only
 Following Environment. Which one has higher data likelihood? Which one
 has higher 
 classification accuracy? Is this surprising? Explain why you see the
 pattern you do.

2. Build a logistic regression model from the Cedergren data,
 using POS, Environment, and Class as the factors. Plot predicted
 versus actual deletion probability for data aggregated by the
 independent variables, and examine the outliers. Describe what you
 see. Compare the outliers to other datapoints. Do the outliers have
 anything in common? Is there anything about these cases that explains
 why these are outliers?

3. Do the beginning part of the Varbrul Cedegren homework, stopping in
 the middle of question 4.  That is, building a basic logistic
 regression model counts as a "one level" analysis.  We will examine
 interaction effects and combining levels later.