Date: Fri, 17 Dec 2010 17:26:48 -0500
Reply-To: Art@DrKendall.org
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: Art Kendall <Art@DrKendall.org>
Organization: Social Research Consultants
Subject: Re: Can I use time and gender as covariates in Ancova?
In-Reply-To: <1292552549667-3308881.post@n5.nabble.com>
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An ANCOVA in this situation is a proper subset of a full factorial
ANOVA.<br>
In the ANCOVA only the main effects of age and gender would be
"controlled"/"accounted" for.<br>
The ANOVA would additionally control for the interaction effects of
<br>
<b>(age by gender), Â (age by time), (gender by time), and (age by
gender by time).</b> <br>
If you only did the ANCOVA part of the overall ANOVA, the 4 effects
above would be pooled in the residual term (i.e., error) making it
larger. The ANCOVA is less powerful that the full ANOVA.<br>
<br>
Think of a test statistic as effect divided by error so the larger
the error the smaller the test statistic.<br>
<br>
Art Kendall<br>
Social Research Consultants<br>
<br>
<br>
<br>
On 12/16/2010 9:22 PM, Bruce Weaver wrote:
<blockquote cite="mid:1292552549667-3308881.post@n5.nabble.com"
type="cite">
<pre wrap="">You can include them as "fixed factors", as Art suggested on Dec 15.
PetterBB wrote:
</pre>
<blockquote type="cite">
<pre wrap="">
Thanks for your comment. Unfortunately, I do not have the actual age, only
age described in 15 different categories.....will this then mean that I
can
not use this as covariates? Do you have any advice?
best
petter
2010/12/16 Bruce Weaver [via SPSSX Discussion] <
<a class="moz-txt-link-abbreviated" href="mailto:ml-node+3308697-636471153-145201@n5.nabble.com">ml-node+3308697-636471153-145201@n5.nabble.com</a><a class="moz-txt-link-rfc2396E" href="mailto:ml-node%2B3308697-636471153-145201@n5.nabble.com"><ml-node%2B3308697-636471153-145201@n5.nabble.com></a>
</pre>
<blockquote type="cite">
<pre wrap="">
</pre>
</blockquote>
<pre wrap="">
</pre>
<blockquote type="cite">
<pre wrap=""> PetterBB wrote:
Hi I thought that with the use of ANCOVA the analysis will adjust the
means
of the outcome variable to what they would be if all subjects scored
equally
on the covariates; gender and age....it this wrong?
best
Petter Bae Brandtzæg.
One of the problems you're running into in this thread is terminological.
In SPSS lingo, covariate = continuous (or scaled) explanatory variable,
and
factor = categorical explanatory variable. So the variables you are
describing as "covariates" are being seen by Art (and others) as
"factors",
because you have age categories, not age as a continuous variable. Do
you
have the actual ages? If so, why are you treating it as categorical?
--
Bruce Weaver
<a class="moz-txt-link-abbreviated" href="mailto:bweaver@lakeheadu.ca">bweaver@lakeheadu.ca</a>
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"When all else fails, RTFM."
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<pre wrap="">
--
Petter Bae Brandtzæg
Ustvedtsvei 14b O871 Oslo
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<pre wrap="">
-----
--
Bruce Weaver
<a class="moz-txt-link-abbreviated" href="mailto:bweaver@lakeheadu.ca">bweaver@lakeheadu.ca</a>
<a class="moz-txt-link-freetext" href="http://sites.google.com/a/lakeheadu.ca/bweaver/">http://sites.google.com/a/lakeheadu.ca/bweaver/</a>
"When all else fails, RTFM."
NOTE: My Hotmail account is not monitored regularly.
To send me an e-mail, please use the address shown above.
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