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Re: st: t test in stata


From   Daljit Dhadwal <ddhadwal@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: t test in stata
Date   Fri, 15 Feb 2013 10:59:32 -0800

Hi Caroline,

I was recently looking into this issue as well: what to do when you
have a mix of paired and unpaired data. I came across the following
paper that you may find helpful. I haven’t implemented the method it
describes though. You may also find the references helpful.

Comparing proportions in overlapping samples: An unpublished paper by
J. Martin Bland and Barbara K. Butland

Link:  http://www-users.york.ac.uk/~mb55/overlap.pdf

Abstract:

Sometimes we want to estimate the difference in proportions between
two groups where some subjects appear in both. We present an approach
which does not require the assumption that the proportions in the
overlapping and non-overlapping samples are the same, and which can be
extended very easily to comparisons of means, odds, etc. The method
has the disadvantage that we need each group to contain some subjects
observed once only. We illustrate the method with an example from the
UK National Child Development Study, and compare the results with
other methods.

Thanks,

Daljit


On Fri, Feb 15, 2013 at 9:55 AM, Caroline Wilson <wilson_cj@hotmail.com> wrote:
> Hi everyone,
>
> I'm trying to determine whether there is a statistically significant change in two proportions between two years, 2005 and 2010. I understand I can use a t-test for this, provided the sample size is large enough. However, I'm not sure whether I should conduct a paired t-test or an unpaired t-test because there are some of the same subjects in both years, but for the most part the two groups represent different subjects.  In this case, is it better to conduct paired t-test because at least some of the same people are in both years?
>
> Any advice would be much appreciated!
> Carrie
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