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Re: st: -ttest- or -prtest-?

From   Richard Williams <>
To   "" <>, "" <>
Subject   Re: st: -ttest- or -prtest-?
Date   Fri, 24 Apr 2009 23:15:30 -0500

At 10:04 PM 4/24/2009, Bert Jung wrote:
Dear Statalisters,

I have an embarrassingly basic question: I am looking at a binary
variable, 0 for "no response" and 1 for "response".  I want to compare
2 groups (treatment/control, also binary).  In this situation is a
-ttest- appropriate or does it have to be a test of proportions

My vague memory of stats 101 suggests to use -prtest- since the
dependent variable is categorical.  But I can interpret my variable as
numerical.  My understanding is that with many numerical categories a
t-test may be ok.  But here the outcome is 0 or 1 and clearly limited
in that range, which calls for a -prtest-.  The two tests would be
quite different in this case, so the choice matters.  Any comments and
views much appreciated.

Use prtest.  See the last page of

In practice, though, it may not matter if you use a t-test instead, at least if the samples are large.

The above is basically a "how to." For more of an explanation, see the last few pages of

Note too that you could also set this up as a 2 by 2 table and do a chi-square test of the model of independence. You can also then use Fisher's exact test, which could be handy if the sample is small. See pp. 4-7 of

Yet another strategy is to do logistic regression, e.g.

logit y x

As add-on: how can I obtain chi2 values for the proportional test when
using complex survey data?

I'm not sure you ever even get chi-square stats when using survey data - you usually get F values or t values, I believe. The first thing that leaps to mind is to do logistic regression, e.g.

svy: logit y x

Richard Williams, Notre Dame Dept of Sociology
OFFICE: (574)631-6668, (574)631-6463
HOME:   (574)289-5227
EMAIL:  Richard.A.Williams.5@ND.Edu

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