st: R: Correlation of repeated baseline measures in sampsi

["Carlo Lazzaro" <carlo.lazzaro@tin.it>]

Helen may want to take a look at Adrian Mander's sampsi_rho (see details
below).
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1 package found (Stata Journal and STB listed first)
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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).

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HTH and Kind Regards,
Carlo
-----Messaggio originale-----
Da: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Per conto di Helen CONNOLLY
Inviato: martedì 22 marzo 2011 22.22
A: statalist@hsphsun2.harvard.edu
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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