Re: Output of 3dGroupInCorr

RWCox

April 01, 2021 11:35AM

Registered: 6 years ago
Posts: 144

It is correct that [0] and [1] are the mean of arctanh(correlation) and the Z-statistic/score of how far that mean is from 0.
However, it is not true that [2] is "the mean of correlation" -- it is the tanh(mean of arctanh(correlation)) -- which is somewhat different.
For example, take 3 values = 0.3, 0.6, 0.9, whose mean is obviously 0.6. However, the following calculation (using AFNI's ccalc program) gives 0.677762 instead:
```
  ccalc 'tanh((atanh(0.3)+atanh(0.6)+atanh(0.9))/3)'
```
Which shows that the "stretching out" action of arctanh gives more weight to larger correlation values.

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