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

["Rodrigo A. Alfaro" <ralfaro76@hotmail.com>]

Also, PASS is another software of Power Analysis for Win. 
You can use it for a free trial period. R.


----- Original Message ----- 
From: <frone@ria.buffalo.edu>
To: <statalist@hsphsun2.harvard.edu>
Sent: Tuesday, September 19, 2006 11:57 AM
Subject: re: st: sample size for correlation, and for Cronbach's alpha


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/


Scroll down to download, select a version, and install

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
Research Institute on Addictions
State University of New York at Buffalo
1021 Main Street
Buffalo, New York 14203

Office:    716-887-2519
Fax:        716-887-2477
E-mail:     frone@ria.buffalo.edu
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"Christopher W. Ryan" <cryan@binghamton.edu> 
Sent by: owner-statalist@hsphsun2.harvard.edu
09/18/2006 06:26 PM
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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
constipation on plain radiographs.

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
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