[R] Automatically Adding Categories

[Jeffrey Edgington]

Greetings,

I have two files which contain responses to a series of multiple  
choice questions.  One
file contains responses before an "intervention" and the other  
contains the responses afterward.

There were three possible responses to each question: D, F, T (for  
Don't Know, False, and True).

I would like to try McNemar's test to determine if there was any  
significant difference between before
and after.

I read the files.  I create tables using:

firstQuestion <- table( PreSurveyData$q1, PostSurveyData$q2)

for example.

The problem is that for several of the questions not all of the  
possible responses appear.  So I get
a table like this:

	T
D	6
F	2
T	12

Which cannot be used in mcnemar.test because it is not a square table  
and does not have enough rows and columns.

Is there some way to specify that R should count the occurrences of  
D, F, and T even though they do not appear
in the Data?  Or some easy way to add the missing columns?

Thank you,

Jeffrey Edgington