Date: Wed, 9 Jul 2008 19:59:24 -0400
Reply-To: "Edgar F. Johns" <efj@discover-tech.net>
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: "Edgar F. Johns" <efj@discover-tech.net>
Subject: Re: insufficient N for factor analysis
In-Reply-To: <58769.20612.qm@web50902.mail.re2.yahoo.com>
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I'm not sure the effort is worth it, but....
You can try to use Dwyer's extension analysis. You start by creating a set
of homogenous item packages or parcels - combine sets of 2-4 items into new
scales by reviewing the item correlations (combine those items with the
highest inter-item correlations). Then, factor analyze the item parcels (you
will have reduced the number of variables in the factor analysis to about
10-15 (instead of 40). Convergence and iterations should behave better.
Rotate and then use the Dwyer extension procedure described in Gorsuch
(1983) Factor Analysis (2nd Ed.) on pages 236-238. Essentially, the factor
solution of the parcels is projected onto the original set of items. You'll
get your factor structure and pattern matrix (if you rotate obliquely) of
the 40 items.
If you need some background on item parceling, you can find out more about
it by searching "item parcels." I know their use is controversial. You can
also check up on Andrew Comrey's work in developing his personality
inventory and Ray Cattell's work.
Edgar
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Discover Technologies
2906 River Meadow Circle.
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-----Original Message-----
From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] On Behalf Of SR
Millis
Sent: Wednesday, July 09, 2008 12:23 PM
To: SPSSX-L@LISTSERV.UGA.EDU
Subject: Re: insufficient N for factor analysis
Comrey & Lee (1992, A first course in factor analysis) give as a guide
sample sizes of:
50 as very poor
100 as poor
200 as fair
300 as good
500 as very good
1000 as excellent for factor analysis.
Tabachnick & Fidell (Using multivariate statistics, 4th ed) recommend at
least 300 cases.
Scott R Millis, PhD, MEd, ABPP (CN,CL,RP), CStat
Professor & Director of Research
Dept of Physical Medicine & Rehabilitation
Wayne State University School of Medicine
261 Mack Blvd
Detroit, MI 48201
Email: smillis@med.wayne.edu
Tel: 313-993-8085
Fax: 313-966-7682
--- On Wed, 7/9/08, Zdaniuk, Bozena <bozena@pitt.edu> wrote:
> From: Zdaniuk, Bozena <bozena@pitt.edu>
> Subject: insufficient N for factor analysis
> To: SPSSX-L@LISTSERV.UGA.EDU
> Date: Wednesday, July 9, 2008, 12:03 PM
> Hello, I was asked to do a factor analysis of 40 variables
> but I only have 70 cases. Needless to say, I had to
> increase iterations to 100 to get the program to converge
> and I still believe that it makes no sense to do a factor
> analysis with less than 2 cases per variable. I was then
> asked to provide a citation for that. Could someone point
> me to a source discussing the minimum case per variable
> requirement for factor analysis that I can cite? Thanks a
> lot.
> Bozena
>
> Bozena Zdaniuk, Ph.D.
> University of Pittsburgh
> UCSUR, 6th Fl.
> 121 University Place
> Pittsburgh, PA 15260
> Ph.: 412-624-5736
> Fax: 412-624-4810
> Email: bozena@pitt.edu
>
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