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st: Logistic Regression_Unequal Ns (outcomes)


From   Chao Yawo <Yawo1964@yahoo.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Logistic Regression_Unequal Ns (outcomes)
Date   Sun, 8 Mar 2009 08:34:14 -0500

Hello, I'm preparing to run a logit model predicting the odds of NOT
testing for an STD.  As you can see from the table below, 4688 (about
98%) of respondents have my outcome of interest (i.e., have not tested
for an STD).  I realized that because of this unequal groupings, all
crosstabulations have higher proportions within the untested category.
 I have a feeling that these could bias my estimates in a way. For
example, given the unequal groupings, I think I am only restricted to
modeling failure to test (the zero outcome), as modeling for ever
tested (1) could lead to unstable estimates.  So my question is  what
possible impact will this have on my model, and what can I do about
it?  Thanks - Chao


(Ever     |
been     |             Type of place of
tested   |                  residence
for STD |        1              2           Total
----------+------------------------------------------------
        0  |      7.973       92.03         100
            |       419         4269        4688
            |
        1  |      62.5          37.5         100
            |       46             30            76
            | -------------------------------------------------
    Total |    8.806         91.19        100
          |       465           4299      4764
-------------------------------
  Key:  row percentages
        number of observations


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