spm_BMS_F.m

function [F_samp,F_bound] = spm_BMS_F (alpha,lme,alpha0)
% Compute two lower bounds on model evidence p(y|r) for group BMS
% 
% FORMAT [F_samp,F_bound] = spm_BMS_smpl_me (alpha,lme,alpha0)
% 
% INPUT:
% alpha     parameters of p(r|y)
% lme       array of log model evidences 
%              rows:    subjects
%              columns: models (1..Nk)
% alpha0    priors of p(r)
% 
% OUTPUT:
% F_samp  -  sampling estimate of <ln p(y_n|r>
% F_bound -  lower bound on lower bound of <ln p(y_n|r>
% 
% REFERENCE: See appendix in
% Stephan KE, Penny WD, Daunizeau J, Moran RJ, Friston KJ
% Bayesian Model Selection for Group Studies. NeuroImage (under review)
%__________________________________________________________________________
% Copyright (C) 2008 Wellcome Trust Centre for Neuroimaging

% Will Penny
% $Id: spm_BMS_F.m 2507 2008-11-30 14:45:22Z klaas $


alpha0 = sort(alpha0);
if alpha0(1) ~= alpha0(end)
    error('Error in function spm_BMS_F: alpha0 should have identical values.')
end
alpha0 = alpha0(1);

a_sum    = sum(alpha);
psi_sum  = psi(a_sum);
psi_diff = psi(alpha) - psi_sum;
gm       = gammaln(alpha);

[s_samp,s_bound] = spm_BMS_F_smpl(alpha,lme,alpha0);

K = length(alpha);
F = 0;
for k = 1:K,
    F = F - (alpha(k) - alpha0)*psi_diff(k) + gm(k);
end
F = F - gammaln(a_sum);

F_bound = F + s_bound;
F_samp  = F + s_samp;

return