t test on each grid point between two 3D matrices (3rd dimension)

Hi guys,

I have two arrays of 25 6x6 matrices each, one per group of subjects. I'd like to perform a Student's t test along the third dimension between the two groups (i.e. each grid point in the first 25 matrices vs each grid point in the second 25 matrices). Ideally, I'd get a 2D matrix output with the t values and another 2D matrix with the p values.

I tried running the code like this:

group1 = randi(100,6,6,25);
group2 = randi(100,6,6,25);

[h,p,stats] = ttest2(group1,group2, 'dim',3,'tail', 'both')

But the stats argument output, which should be a struct with t values, df and sd if I'm not mistaken, is this:

stats(:,:,1) =

  -24.8068  -11.8843  -38.5223  -20.2231  -18.5612  -35.3428
  -27.7352    0.2262   -5.8083  -13.9098  -21.7295  -10.9928
   -6.6216   -8.7296  -14.5600  -18.9004  -13.2651  -10.9495
  -19.2140  -14.6268  -30.3848  -16.6755  -15.2337  -15.8925
  -21.4499  -30.1524  -13.1650   -2.7901  -17.5884  -28.3383
  -16.7520  -17.1620  -15.6470   -0.8119  -14.6905   -8.9892


stats(:,:,2) =

   10.0868   14.9243   -5.0777   11.7431   16.3212   -5.5372
    6.1352   30.4138   26.5283   17.5098   10.1295   19.3928
   26.3016   25.3696   20.4800   17.7804   20.7051   22.1495
    9.6940   17.0268   -1.7752   16.9155   15.3937   17.8125
   14.6499    1.9924   22.2850   31.1101   14.5484    5.4583
   13.9520   14.3620   18.0470   31.6119   18.1305   22.1092

I don't understand why it's giving me two matrices and not a struct.

If I don't specify 'dim' in the code, it gives me 25 matrices for stats!

I'm running this on Matlab R2021a.

Any ideas? Thanks a lot in advance!

Answer 1

[Scott MacKenzie]

You are using the wrong form of ttest2. If you want to get the stats structure, change

[h,p,stats] = ttest2(group1,group2, 'dim',3,'tail', 'both')

to

[h,p,~,stats] = ttest2(group1,group2, 'dim',3,'tail', 'both')

[Francesca Fusina]

Thanks a lot Scott! Worked great!