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Re: st: R: Correlation of repeated baseline measures in sampsi


From   Helen CONNOLLY <[email protected]>
To   [email protected]
Subject   Re: st: R: Correlation of repeated baseline measures in sampsi
Date   Wed, 23 Mar 2011 20:05:07 +0100

Thanks Carlo,

That is helpful information (and I found other useful commands through the link), but it doesn't quite answer my question. sampsi_rho will calculate a sample size if you know rho, but how do you find rho (r0, r1, or r01)?

Can it be as simple as :
corr value id if baseline
(where "value" is the repeated measure, "id" is the patient id, and baseline is a dummy to indicate a baseline measure)?
What if the baseline measures are monthly measures?

Thanks


On 3/23/2011 7:49 AM, Carlo Lazzaro wrote:
Helen may want to take a look at Adrian Mander's sampsi_rho (see details
below).
..........................................................................
search for sampsi_rho                                              (manual:
[R] search)
----------------------------------------------------------------------------
------------

         Keywords:  sampsi_rho
           Search:  (1) Official help files, FAQs, Examples, SJs, and STBs
                    (2) Web resources from Stata and from other users


Search of official help files, FAQs, Examples, SJs, and STBs

Web resources from Stata and other users

(contacting http://www.stata.com)

1 package found (Stata Journal and STB listed first)
----------------------------------------------------

sampsi_rho from http://fmwww.bc.edu/RePEc/bocode/s
     'SAMPSI_RHO': module to compute sample size for a Pearson correlation /
     	sampsi_rho calculates either the sample size or power for a /
Pearson
     	correlation. The formula for sample size is / ((Za+Zb)/(delta))2 + 3
where delta is the difference between test value and the null value, 	Za
is the signficance Z-value and Zb is the power Z-value. The 	correlations
are transformed using the Fisher z transformation
1/2ln((1+rho)/(1-rho)) this z is normally distributed with a standard
deviation of 1/sqrt(N-3).

............................................................................

HTH and Kind Regards,
Carlo
-----Messaggio originale-----
Da: [email protected]
[mailto:[email protected]] Per conto di Helen CONNOLLY
Inviato: martedì 22 marzo 2011 22.22
A: [email protected]
Oggetto: st: Correlation of repeated baseline measures in sampsi

Hello,

I'm using the sampsi command to calculate sample size for a two-sample
study with 12 monthly baseline measurements and 12 monthly follow-up
measurements.  I have data from a previous study and can calculate mean
and standard deviations for both samples.

My question is, how do I calculate correlation of baseline measurements
(r0), correlation of follow-up measurements (r1), and correlation
between baseline and follow-up measurements (r01)?  These require a
single measure (not a covariance matrix).  In all the references I have
seen, there are statements like "correlation of baseline measurements
calculated from previous study", but I see no reference as to how this
is done.  Can someone please help?

Thank you,
Helen Connolly

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