tapas_beta_obs.m

function [logp, yhat, res] = tapas_beta_obs(r, infStates, ptrans)
% Calculates the log-probability of responses representing probabilities on the unit interval
%
% --------------------------------------------------------------------------------------------------
% Copyright (C) 2013-2016 Christoph Mathys, TNU, UZH & ETHZ
%
% This file is part of the HGF toolbox, which is released under the terms of the GNU General Public
% Licence (GPL), version 3. You can redistribute it and/or modify it under the terms of the GPL
% (either version 3 or, at your option, any later version). For further details, see the file
% COPYING or <http://www.gnu.org/licenses/>.

% Transform nu-prime to its native space
nupr = exp(ptrans(1));

% Initialize returned log-probabilities, predictions,
% and residuals as NaNs so that NaN is returned for all
% irregualar trials
n = size(infStates,1);
logp = NaN(n,1);
yhat = NaN(n,1);
res  = NaN(n,1);

% Predictions or posteriors?
if strcmp(r.c_prc.model,'tapas_rw_binary')
    mu = tapas_sgm(infStates(:,1), 1);
else
    mu = infStates(:,1,1); % Default: predictions (ie, mu1hat)
    if r.c_obs.predorpost == 2
        mu = tapas_sgm(infStates(:,2,3), 1); % Alternative: posteriors (ie, sgm(mu2))
    end
end

% Special cases
if strcmp(r.c_prc.model,'hgf_whichworld')
    mu = tapas_sgm(infStates(:,2,1,3), 1);
end
if strcmp(r.c_prc.model,'ph_binary')
    mu = infStates(:,2);
end

% Weed irregular trials out from inferred states and responses
mu(r.irr) = [];
y = r.y(:,1);
y(r.irr) = [];

% y has to be in the *open* unit interval
%y(y==0) = 1e-4;
%y(y==1) = 1-1e-4;
y = 0.95.*(y-0.5)+0.5; % Shrink all y values toward 1/2 by a factor of 0.95

% Nu is nu-prime plus two (sometimes)
%nu = nupr+2;
nu = nupr;

% Calculate alpha and beta from mu and nu
al = mu.*nu;
be = nu - al;

% Calculate log-probabilities for non-irregular trials
reg = ~ismember(1:n,r.irr);
logp(reg) = log(betaDens(y,al,be));
yhat(reg) = mu;
res(reg) = y-mu;

end

function p = betaDens(x,alpha,beta)
% Check whether x is in the unit interval
if any(x(:)<0) || any(x(:)>1)
    error('tapas:hgf:BetaObs:ArgNotInUnitIntrv', 'Error: first argument to betaDens must be in the unit interval.');
end
% Check whether alpha and beta are greater than 0
if any(alpha(:)<0) || any(beta(:)<0)
    error('tapas:hgf:BetaObs:AlphaOrBetaNeg', 'Error: alpha and beta have to be non-negative.');
end
% Calculate beta density
p = gamma(alpha+beta)./(gamma(alpha).*gamma(beta)).*x.^(alpha-1).*(1-x).^(beta-1);
end