Thank yot for your excellent detailed explanation!
Sigalit.


On 10/24/07, Ted Harding <Ted.Harding@manchester.ac.uk> wrote:
>
> On 24-Oct-07 08:51:30, sigalit mangut-leiba wrote:
> > hello,
> > I want to do a single Imputation method on binary data set.
> > Is it possible to use imp.cat from CAT package?
> > I have a problem defining "theta" when data is binary.
> > Do you know any references on the subject?
> > Thank you,
> > Sigalit.
>
> There should be no problem in principle. However, you must
> make sure to do things in the correct sequence.
>
> 1. Make sure your data are in a matrix, not a dataframe,
>   where the rows correzspond to cases and the columns
>   correspond to variables. (In passing: you must of
>   course have at least two variables). If your data are
>   in a dataframe Y in the first instance, first do
>
>   X <- as.matrix(Y)
>
> 2. Make sure that the binary outcomes are coded as 1 and 2,
>   not 0 and 1. Missing values should be coded as NA. If
>   the outcomes are 0 and 1, then do
>
>   X <- X+1
>
> 3. Now do the preliminary step, which creates a specially
>   formatted representation of the data, missing value
>   patterns, etc. (and of course first load the 'cat' package):
>
>   library(cat)
>   s <- prelim.cat(X)
>
>   where X is your data matrix as in (1) and (2).
>
> 4. Next, the maximum-likelihood estimation:
>
>   t <- em.cat(s)
>
> 5. Now you can embark on as many imputations as you wish,
>   alternating between da.cat() and imp.cat():
>
>   t <- da.cat(s,t)
>   imp.1 <- imp.cat(s,t)
>
>   t <- da.cat(s,t)
>   imp.2 <- imp.cat(s,t)
>
>   ...
>
> More details can be found in the documentation for the
> functions listed by
>
>   library(help=cat)
>
> e.g. ?prelim.cat, ?em.cat, ?da.cat, ?imp.cat, etc.
>
> The above is for a straightforward imputation of missing
> values in a dataset of caregorical variables, and does not
> incorporate any special modelling considerations. See also:
> ?ecm.cat and other functions.
>
> Hoping this helps,
> Ted.
>
> --------------------------------------------------------------------
> E-Mail: (Ted Harding) <Ted.Harding@manchester.ac.uk>
> Fax-to-email: +44 (0)870 094 0861
> Date: 24-Oct-07                                       Time: 10:59:01
> ------------------------------ XFMail ------------------------------
>

	[[alternative HTML version deleted]]

