Hi Nicolas
The "common random effects design matrix" expression means that all subjects in your prospective study are expected in advance to have the same random effects design matrix (Zi). If you assume a linear trajectory over time and a two-year study with repeated measurements every six months then this matrix can be:
1 0 1 0.5 1 1 1 1.5 1 2
In this case you are considering both intercept and time as random effects. ZiCol is the column of the above matrix that you are interested in (usually column 2, slope of change over time).
To compute sample size you also need estimates of the random effects covariance matrix (Dhat), within-subject measurement error, effect size, and drop-out rate. If your study is about Alzheimer-related hippocampal volume or cortical thickness variations you can use our sample ADNI data to compute those values (ADNI791_Hipp_and_Entorh.mat). Eg.
lme_plannedSampleSize(Zi,2,total_hipp_vol_stats.Dhat,total_hipp_vol_stats.phisqhat,1,.2,.8,0.05)
Alternatively, you can perform a pilot longitudinal study with a small sample or you can use information from previous studies that are similar to the study that you are planning to run, or you can even analyse some public available longitudinal data with lme to compute those values.
We performed sample size estimations for the study described in our paper in the wiki page:
http://surfer.nmr.mgh.harvard.edu/fswiki/LinearMixedEffectsModels
It may be useful for you to take a look at it.
Best -Jorge
De: nicolas guizard n.guizard@gmail.com Para: freesurfer@nmr.mgh.harvard.edu Enviado: Jueves 11 de abril de 2013 14:28 Asunto: [Freesurfer] Fwd: Sample size estimation from LME models
Hi, I am new to Freesurfer and the Linear Mixed Effects (LME) Models packages. And I would like to compute sample size from volumetric measures for 2 groups.
I followed the longitudinal example provided in the wiki (http://surfer.nmr.mgh.harvard.edu/fswiki/LinearMixedEffectsModels#a.29Univar...), with the ADNI example. But I am not sure what to use for the "common random effect design matrix" Zi and "column" ZiCol with the lme_plannedSampleSize(Zi,ZiCol,Dhat,phisqhat,effsz,dr,pw,alpha,gr_pr) function:
Here is what I do in matlab:
load('ADNI791_Hipp_and_Entorh.mat')
total_hipp_vol_stats = lme_fit_EM(X_Hipp,[1,2],Y(:,1)+Y(:,2),ni);
lme_plannedSampleSize(total_hipp_vol_stats.X,[1,2],total_hipp_vol_stats.Dhat,total_hipp_vol_stats.phisqhat,1,.2,.8,0.05)
ans =
1.0e+07 *
1.3584 0.0154 0.0154 0.0100
Is that right?
Regards, Nicolas _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
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