Hi Pinar,
It is my understanding that you would like to define custom starting points for your curve - fitting model for different participants. To do this, we can create an n by 3 matrix startPts (n being the number of participants) containing the parameters a, m and s in that order (i.e. the i - th row of startPts is the start point for the i - th participant). Then, we can use a loop to create n different fit objects which have different starting points based on the vectors extracted from startPts. I have a code snippet below that you may find useful.
%startPts is the n - by - 3 matrix containing the start points for each of the 'n' participants;
ft = fittype( 'a*(.5*(1+erf((x-m)/(s*sqrt(2)))))', 'independent', 'x', 'dependent', 'y' );
opts = fitoptions( 'Method', 'NonlinearLeastSquares' );
opts.Display = 'Off';
opts.MaxFunEvals = 1000;
s = cell(n,1); %store the n fitobjects for plotting
for i = 1:n
opts.StartPoint = startPts(i,:);
s{i} = fit(x,y,ft,opts);
end
