see inserted comments.
> Date: Mon, 22 Oct 2012 21:24:20 -0700
> From:
[hidden email]> Subject: Alpha and mean inter item correlation
> To:
[hidden email]>
> Hi
> I am currently using SPSS. I am a novice. I have two set of survey done in
> Stage 1 of my research and after 3 months the same survey was done in Stage
> 2.
Pre-post pairs allow you to look at consistency across time,
in addition to looking at change.
>
> I did an exploratory factor analysis but that was thrown out because I have
> grouped the variables in a theme which my Supervisor said that cannot be
> done. EFA can only be done with about 10 to 20 variables and not 4 to 5
> variables.
Well, you can *do* FA with most any N, if you are ready to cope
with the consequences. Too many variables make for a non-robust
solution -- The rule orf thumb of 10 cases per variable suggests 15
should be good. But if there is strong structure, you might have a
good solution with 30. Or a sloppy solution with your 69 vars.
I've always looked at new data with factor analysis, just for my
own information... to confirm, for instance, that the data *do* have
the sort of correlations that go along with valid data entry of a
scale on a topic.
I don't know why your supervisor says it "can't be done" with 5 vars,
unless he is sure that you don't want to merely report that they do
or don't cohere. (Try it anyway, and see what you can say about it.)
>
> Now he told me that I need to do an alpha and mean inter item correlation. I
> can navigate and manage to do this, alpha and mean inter item correlation.
> My question is do I put both stages of the survey or only 1 stage of the
> survey result? There are 69 variables and my participants = 157.
>
> The result as shown from the output was participants 314 (this means they
> took both stages/I fed in both stages) with 69 varibles. Does it matter?
> whether its 1 stage or 2 stages, will the result varies?
> ...
I won't say "never do it", but it is rare to want the same cases entered
more than once for anything you will publish. Results are usually
similar if you do the two periods separately, even if there was an
active intervention. But you did not say what sort of items these
are, or if there were special circumstances at either period. Patient
scores at "intake" may differ a lot from scores 3 months later.
With 69 variables, I have always looked at subscales, either from
the result of my factoring or from a-priori logic-- Use published data,
or "expert opinion" (whoever is available that you can tout as expert)
to pragmatically define some relevant latent variables.
--
Rich Ulrich