Dear SPM experts,
I have a very basic question, not specific to neuroimaging.
I have statistical parameters for few samples: M1, M2, M3..., SD1, SD2, SD3..., N1, N2, N3... (mean, standard deviation and the size of each sample). I dont have original data. I want to estimate total M
and SD. Estimation of total mean seems easy: (M1*N1+M2*N2+M3*N3... ) N1+N2+N3...
However I have a problem with total SD. With a model data sample the formula: sqrt( ((N1-1)*SD1^2+(N2-1)*SD2^2+(N3-1)*SD3^2) / (N1+N2+N3-3) ) does not work. Perhaps the problem is that valiance between meas (M1 M2 and M3) is not taken into account.
Thank you very much for nay help!
Sincerely yours,
Vladimir
