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help > RE: atan(r) values of second level results
Jul 8, 2012 10:07 PM | Alfonso Nieto-Castanon - Boston University
RE: atan(r) values of second level results
Hi Crystal,
I would not be worried. The beta values in these analyses (Fisher transformed correlation coefficient values, often called Z-values) are not the same as the normal distribution standard scores values (often called z-values), the former are measures of effect size that directly relate to the correlation coefficient units, while the latter are measures of statistical strength that directly relate to the T- values of your results. If you want to transform z- values to equivalent T-statistics, simply use the transformation:
T=tinv(normcdf(z),dof);
(for example, for a simple t-test with 20 subjects, where dof=19, a z-value of 3 corresponds approximately to a T-value of 3.45).
If you want to transform your Fisher transformed Z-values to correlation coefficient values simply use the transformation:
r=tanh(Z);
(for example, a Z-value of 3 would correspond to a very unrealistic correlation coefficient of 0.995; Z-values in the .05 to .30 range look perfectly fine to me -they roughly correspond to r-values in the .05 to .29 range)
Hope this clarifies
Best
Alfonso
Originally posted by Crystal Goh:
I would not be worried. The beta values in these analyses (Fisher transformed correlation coefficient values, often called Z-values) are not the same as the normal distribution standard scores values (often called z-values), the former are measures of effect size that directly relate to the correlation coefficient units, while the latter are measures of statistical strength that directly relate to the T- values of your results. If you want to transform z- values to equivalent T-statistics, simply use the transformation:
T=tinv(normcdf(z),dof);
(for example, for a simple t-test with 20 subjects, where dof=19, a z-value of 3 corresponds approximately to a T-value of 3.45).
If you want to transform your Fisher transformed Z-values to correlation coefficient values simply use the transformation:
r=tanh(Z);
(for example, a Z-value of 3 would correspond to a very unrealistic correlation coefficient of 0.995; Z-values in the .05 to .30 range look perfectly fine to me -they roughly correspond to r-values in the .05 to .29 range)
Hope this clarifies
Best
Alfonso
Originally posted by Crystal Goh:
Hi there,
I performed seed to voxel fc with bivariate correlation and produced second level maps per condition per seed using a within-subjects design.
when I open the saved .nii masks in mricron, or when I plot beta values in SPM, the atan(r) values are SUPER low, they range from 0.05 to 0.3. Most papers with fc threshold their z-maps above say z=3, but this is impossible for me.
I wanted to know if I should be worried, since the t-values of significant clusters have no bearing on the strength of the correlations themselves.
Any help appreciated. Thanks!
crystal
I performed seed to voxel fc with bivariate correlation and produced second level maps per condition per seed using a within-subjects design.
when I open the saved .nii masks in mricron, or when I plot beta values in SPM, the atan(r) values are SUPER low, they range from 0.05 to 0.3. Most papers with fc threshold their z-maps above say z=3, but this is impossible for me.
I wanted to know if I should be worried, since the t-values of significant clusters have no bearing on the strength of the correlations themselves.
Any help appreciated. Thanks!
crystal
Threaded View
| Title | Author | Date |
|---|---|---|
| Crystal Goh | Jun 2, 2012 | |
| Alfonso Nieto-Castanon | Jul 8, 2012 | |
| Patrick McConnell | Dec 20, 2014 | |
| Alfonso Nieto-Castanon | Dec 21, 2014 | |
| Patrick McConnell | Dec 21, 2014 | |
| Alfonso Nieto-Castanon | Dec 24, 2014 | |
| Patrick McConnell | Dec 24, 2014 | |
| Patrick McConnell | Jan 8, 2015 | |
| Alfonso Nieto-Castanon | Dec 27, 2014 | |
| Patrick McConnell | Jan 7, 2015 | |
| Alfonso Nieto-Castanon | Jan 15, 2015 | |
| Patrick McConnell | Jan 22, 2015 | |
| Michael King | Dec 16, 2014 | |
| Alfonso Nieto-Castanon | Dec 17, 2014 | |
| Michael King | Dec 17, 2014 | |
| Michael King | Dec 17, 2014 | |