Dear AFNI experts,
I am transforming F- and t-statistic maps to z-statistic maps using 3dcalc. In some cases (when df1=1) I think that the z-statistic map resulting from the transformation of the F-statistic should be the same as the z-statistic map resulting from the transformation of the t-statistic. However, I tend to get slightly different results in such cases. I am wondering if this is due to numerical issues or if I make a mistake. Below the AFNI commands:
3dinfo stats.sub-CAS0001_REML+tlrc[0]
-- At sub-brick #0 'Full_Fstat' datum type is float: 7.54451e-11 to 65.7991
statcode = fift; statpar = 1 247
3dinfo stats.sub-CAS0001_REML+tlrc[2]
-- At sub-brick #0 'activation_stimulus#0_Tstat' datum type is float: -4.76188 to 8.11167
statcode = fitt; statpar = 247
3dcalc -a stats.sub-CAS0001_REML+tlrc[0] -expr 'cdf2stat(stat2cdf(a, 4, 1, 247, 0), 5, 0, 0, 0)' -prefix standardized_stats/zstat_from_Fstat.nii
3dinfo standardized_stats/zstat_from_Fstat.nii
-- At sub-brick #0 'Full_Fstat' datum type is float: -4.34628 to 7.5404
statcode = fift; statpar = 1 247
3dcalc -a stats.sub-CAS0001_REML+tlrc[2] -expr 'fitt_t2z(a, 247)' -prefix standardized_stats/zstat_from_tstat.nii
3dinfo standardized_stats/zstat_from_tstat.nii
-- At sub-brick #0 'activation_stimulus#0_Tstat' datum type is float: -4.65303 to 7.63014
statcode = fitt; statpar = 247
As you see, the minimum values in the z-statistic maps are -4.34628 and -4.65303, not quite the same. Is it a numerical issue?
Sorry for bothering you!
Best wishes,
Wiktor Olszowy
