Calculate the mean of two dimensions in a 3D array

I have a 3D array and I want to calculate the mean of the first two dimensions along the 3rd dimension but I am getting the following error

"Assignment has more non-singleton rhs dimensions than non-singleton subscripts".

My code looks as follows

        MeanMatrix = zeros(1,1,30);
        ImageROI = data(50:55,100:105,:);
        for i=1:30
            MeanMatrix(:,:,i) = mean(ImageROI(:,:,i));       
        end

Answer 1

[Guillaume]

mean on a 2D matrix (your imageROI(:, :, i)) will return a vector. You can't assign a vector to a scalar (which is what MeanMatrix(:, :, i) is since it has 1 row and 1 column).

In any case, the loop is completely unnecessary:

MeanMatrix = mean(mean(ImageROI, 1), 2);

Answer 2

[KL]

If you want the mean of all rows in every page(3rd dimension)

 mean(ImageROI,1)

for columns in every page

 mean(ImageROI,2)

and mean of pages,

 mean(ImageROI,3)

if you're looking to calculate the mean of all elements in a page, mean2 would be something

 mean2(ImageROI(:,:,pageNo)) %with a loop

without a loop, probably with reshape,

 mean(reshape(ImageROI,[],size(ImageROI,2),1))