Date:         Tue, 26 Jan 2010 07:10:11 -0800
Reply-To:     Bruce Weaver <bruce.weaver@hotmail.com>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Bruce Weaver <bruce.weaver@hotmail.com>
Subject:      Re: t-test with small n
In-Reply-To:  <4E70ADC2-5C5A-4C1D-B18F-7D033478ABF2@ifi.uzh.ch>
Content-Type: text/plain; charset=us-ascii

aschoff wrote:
>
> Hello,
>
> we are dealing with survey data for two independent groups we would
> like to compare. Due to missing values we have very small and unevenly
> distributed data for some parts of the survey (e.g. 5 responses for
> one group, 8 responses for the control group, answers on likert
> scales). We have a discussion whether a t-test can be used to compare
> these two groups.
>
> We assume that is is not appropriate to use the t-test but we would
> like to understand the exact nature of the problem. We are aware of
> the requirments for the t-test like normal distribution, equal
> variances etc. We are also aware that this test has a low test-power,
> however when we try it out the test turns out to be significant.
>
> What would be the correct argument to refuse using the t-test for this
> situation (if it is to be refused?). Is it just because we are having
> difficulties to prove that we meet the requirements? What would you
> consider a lower boundary that justifies to use a t-test? Would it be
> more appropriate to use a non parametric test or is it just impossible
> to show a systematic difference with a small N like this?
>
> Thanks a lot for your input.
> Robinson
>
>

You might find this BMJ note useful.

  http://www.bmj.com/cgi/content/full/338/apr06_1/a3166



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Bruce Weaver
bweaver@lakeheadu.ca
http://sites.google.com/a/lakeheadu.ca/bweaver/
"When all else fails, RTFM."

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