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

[Helen CONNOLLY <h.connolly@numericable.lu>]

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: 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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