Dear Donald,

Making this change should have a trivial impact on the results - it is just to regularize
the inversion. You can regard the speye(length(C))*exp(-16) as a very uninformative
augmentation of the prior precision

I have just checked our development code and we do indeed use  speye(length(C))*exp(-16).
In future, we will use spm_inv - see below.

With very best wishes,

Karl





function X = spm_inv(A,TOL)
% inverse for ill-conditioned matrices
% FORMAT X = spm_inv(A)
%
% A  - matrix
% X  - inverse
%
% This routine simply adds a small identity matrix to A and calls inv.m
%__________________________________________________________________________
% Copyright (C) 2008 Wellcome Trust Centre for Neuroimaging
 
% Karl Friston
% $Id: spm_inv.m 4098 2010-10-22 19:46:28Z karl $
 
% check A
%--------------------------------------------------------------------------
[m,n] = size(A);
if isempty(A), X = sparse(n,m); return, end
 
% tolerance
%--------------------------------------------------------------------------
TOL = max(eps(norm(A, 'inf' ))*max(m,n),exp(-32));
 
% inverse
%--------------------------------------------------------------------------
X   = inv(sparse(A) + speye(m,n)*TOL);





At 04:03 17/03/2011, MCLAREN, Donald wrote:

Karl,

I've noticed a number of posts recently about changing the following line in spm_PEB.m to avoid warning messages:
line 371 from C  = inv(C + speye(length(C))*exp(-32)); to C  = inv(C + speye(length(C))*exp(-16));

(1) Does this change make a large impact on the analyses?
(2) Are there plans to implement the change in future versions?
(3) Should it be changed?

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School
Office: (773) 406-2464
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