[R] Automatically Adding Categories

[David Winsemius]

On Jun 22, 2009, at 7:56 PM, Jeffrey Edgington wrote:

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

Missing cells added:
	T   D  F
D	6   0  0
F	2   0  0
T	12  0  0

>
> 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?


You might want to exercise appropriate care regarding the validity  
concerns, since you obviously have quite a few zero entries.

-- 

David Winsemius, MD
Heritage Laboratories
West Hartford, CT