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March 13, 2018 09:39AM
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
Subject Author Posted

3dcalc transformations of the statistic maps

wo222 March 13, 2018 09:39AM

Re: 3dcalc transformations of the statistic maps

Gang March 13, 2018 05:57PM

Re: 3dcalc transformations of the statistic maps

wo222 March 13, 2018 07:40PM

Re: 3dcalc transformations of the statistic maps

Gang March 14, 2018 11:03AM



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