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Re: st: ## Interaction syntax

From   "Michael N. Mitchell" <>
Subject   Re: st: ## Interaction syntax
Date   Tue, 11 Jan 2011 15:41:27 -0800

Dear Robin

That is very peculiar, because indeed the two examples you provided should be identical. The only thing I can think of that could cause an issue is if you had ill behaved data, and somehow these two techniques were dealing with the ill behaved data differently. (An example would be multi-collinearity or empty cells). Are you able to provide examples of output where these two techniques diverge (showing the model that converged). Perhaps the model that converged might give a clue?

Best regards,

Michael N. Mitchell
Data Management Using Stata      -
A Visual Guide to Stata Graphics -
Stata tidbit of the week         -

On 2011-01-11 3.28 PM, Robin Jeffries wrote:
I was under the impression that for a categorical variable 'wave' with
3 levels (0, 1, 2)
and another binary indicator variable 'group' (0,1) then the following
statements are the same:

1) xtlogit (other covar)

2) xtlogit wave1 wave2 group wg1 wg2 (other covar)

where wave1, wave2, wg1, wg2 are the manually created indicators and
interactions for wave and wave*group respsectivly

For most outcomes I am using this with, they produce the exact same
results. However there have been some instances where using the second
method results in a model that won't converge, but the first will.

Is there an explanation for this?

Robin Jeffries
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