# re: st: sample size for correlation, and for Cronbach's alpha

 From [email protected] To [email protected] Subject re: st: sample size for correlation, and for Cronbach's alpha Date Tue, 19 Sep 2006 11:57:34 -0400

```Christopher,

Regarding your first question, I don't have a Stata answer, but you could
try a free power program called G*Power, which can do sample size
computations for correlations.  Find it at:

http://www.psycho.uni-duesseldorf.de/aap/projects/gpower/

To get instructions on using the program, scroll down to help, click the
hypertext, and then click on statistical tests.

A bit of a clunky DOS program, but it's easy to use and the price is
right.  But there are many other commercial programs.

Mike Frone

****************************************************************
Michael R. Frone, Ph.D.
Senior Research Scientist
State University of New York at Buffalo
1021 Main Street
Buffalo, New York 14203

Office:    716-887-2519
Fax:        716-887-2477
E-mail:     [email protected]
Internet: http://www.ria.buffalo.edu/profiles/frone.html
****************************************************************

"Christopher W. Ryan" <[email protected]>
Sent by: [email protected]
09/18/2006 06:26 PM
[email protected]

To
Statalist <[email protected]>
cc

Subject
st: sample size for correlation, and for Cronbach's alpha

I am attempting to create a summated rating scale to detect functional
fecal retention (ffr) in children.  The scale contains 15 items (v1-v16,
but no v10 (which is an unrated distractor)).  12 Items are rated 0-2; 3
items are rated 0-4.  I had planned to sum the scores on the individual
items; sum can range 0-36.  Higher scores mean more constipated.

I hope to correlate the score on the scale with visual assessments of

Two questions come to mind:

Does Stata have a sample size routine for correlations?

How does sample size relate to Cronbach's alpha?  I've been told that
roughly ten subjects per item would yield a reasonable sample size for
reliability testing; this would be 150 in my case.  I have little hope
of recruiting more than 50-60 subjects for this initial study, unless I
extend it to a longer duration, which I'd like to avoid.

To my knowledge, no such survey instrument exists, so these are
relatively uncharted waters.  I've only tried out the survey on 8
subjects, just to see if they could understand the questions:

.slist v1-v16

v1  v2  v3  v4  v5  v6  v7  v8  v9  v11  v12  v13  v14  v15  v16
0   0   0   0   0   0   0   0   0    0    0    0    1    2    0
1   0   0   0   0   0   0   0   0    0    0    0    1    1    1
0   0   0   0   0   0   1   0   0    0    0    0    1    3    .
1   0   0   0   0   1   0   0   0    0    0    0    2    2    1
2   1   1   2   0   1   2   1   2    1    0    2    3    2    2
1   0   0   1   0   0   1   0   2    0    1    1    3    1    0
2   2   2   2   1   1   2   2   0    0    0    2    1    .    .
2   1   1   1   2   1   2   1   1    1    2    2    2    2    0

I feel silly calculating it for just 8 cases, but alpha for the
instrument as a whole was

.  alpha v1-v16

Test scale = mean(unstandardized items)

Average interitem covariance:      .274784
Number of items in the scale:           15
Scale reliability coefficient:      0.9211

What conclusions could I draw from this, about the likelihood of
obtaining a respectable alpha with less than 150 subjects?  Or is that a
nonsensical question?

Thanks.

--Chris
--
Christopher W. Ryan, MD
SUNY Upstate Medical University Clinical Campus at Binghamton
and Wilson Family Practice Residency, Johnson City, NY
cryanatbinghamtondotedu
GnuPG and PGP public keys available at http://pgp.mit.edu

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divide the work and give orders. Instead, teach them to yearn for the
vast and endless sea."  [Antoine de St. Exupery]
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