Re: Is ANCOVA being used correctly?

Posted by Bruce Weaver on Oct 01, 2013; 11:16am
URL: http://spssx-discussion.165.s1.nabble.com/Is-ANCOVA-being-used-correctly-tp5722258p5722341.html

Try the link given in Rich Ulrich's post:

   http://w3.psychology.su.se/staff/marlar/Psych_Med_2004.pdf

Ismail Noor wrote

When I click on this link>>>>>>>>>>>>>>>>>>>
"I am writing an evaluation of a research paper, which can be found here:
> http://w3.psychology.su.se/staff/mar...h_Med_2004.pdf"................................

I get this:  

Not Found
The requested URL /staff/mar...h_Med_2004.pdf was not found on this server.

 
 
Ismail K. Noor, Ph.D.,
Data Group Consultant, LLC
1335 Culver Avenue
Dearborn, Michigan   48124
[hidden email]
[phone]
[phone]



________________________________
 From: Bruce Weaver <[hidden email]>
To: [hidden email] 
Sent: Monday, September 30, 2013 6:29 PM
Subject: Re: Is ANCOVA being used correctly?
 

This post reminded me of some comments made in an older thread that started
off being about the Chow test, but ended up being more about ANCOVA-like
models in general.  Here are some comments I made followed by comments from
Richard Ristow.

http://spssx-discussion.1045642.n5.nabble.com/Chow-test-to-compare-regression-on-two-groups-tp2839448p2839846.html

http://spssx-discussion.1045642.n5.nabble.com/Chow-test-to-compare-regression-on-two-groups-tp2839448p2839868.html


HTH.



mithrauko wrote
> Hi! I am having a serious horrible time with an ANCOVA issue, and I would
> be immensely grateful for any help.
>
> I am writing an evaluation of a research paper, which can be found here:
> http://w3.psychology.su.se/staff/mar...h_Med_2004.pdf
>
> Basically, they are comparing a) a group of depressed individuals with a
> group of non-depressed individuals and b) different types of depressed
> individuals (eg, dysthymia, minor depression etc) with each other an with
> the non-depressed individuals. They use ANCOVAs. They use gender as a
> covariate in all these.
>
> Ok, so, problem: Basically, in Field (Discovering statistics... p397-9)
> and also Miller and Chapman (2001) as cited in Field
> (http://homepages.gold.ac.uk/aphome/ancova.pdf), it says 'analysis of
> covariance cannot tell us how groups would differ if they did not differ
> on the covariate' and that 'mistakenly, investigators frequently turn to
> ANCOVA in hopes of 'controlling for' group differences on the covariate'
> etc etc (both quotes from M&C).
>
> Therefore, they both seem to suggest that you can't use an ancova if there
> are differences between groups as to the covariate.
>
> Questions:
>
> 1) In my Paper 2, it says that gender differs across groups (according to
> a chi-squared test), but they still entered gender as the covariate. Is
> this the wrong analysis method to use then??
>
> 2) To complicate matters, it goes on about how the covariate must be
> independent of the treatment effect etc, but I don't exactly have a
> treatment effect as it's just measuring scores on tests, and so I got
> confused. Field puts as an example: anxiety is correlated with depression
> (anxious people tend to be more depressed), so if comparing anxious with
> non anxious people, the anxious group might be more depressed. Might want
> to enter depression as covariate to find 'pure' effect of anxiety, but you
> can't as anxiety and depression share variance. I can understand this
> example, but unfortunately I can't relate it to my Paper 2 as I am
> confused. Gender is correlated with depression (or at least, females more
> likely to be depressed, is that the same thing?)
>
> - does this mean 'gender shares some of its variance with depression'?
>
> Moreover it seems that if gender affects test scores, which it might, then
> it also can't be used as a covariate..?
>
> Lastly, some papers say you can only use a continuous variable as a
> covariate, but gender is obviously categorical.
>
> Essentially, have they used ANCOVAs incorrectly, then? What should they
> have done instead, if so?
>
> REALLY CONFUSED!! Any help would be much much much appreciated.
>
> Isabel





-----
--
Bruce Weaver
[hidden email]
http://sites.google.com/a/lakeheadu.ca/bweaver/

"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.

--
View this message in context: http://spssx-discussion.1045642.n5.nabble.com/Is-ANCOVA-being-used-correctly-tp5722258p5722337.html
Sent from the SPSSX Discussion mailing list archive at Nabble.com.

=====================
To manage your subscription to SPSSX-L, send a message to
[hidden email] (not to SPSSX-L), with no body text except the
command. To leave the list, send the command
SIGNOFF SPSSX-L
For a list of commands to manage subscriptions, send the command
INFO REFCARD