function err = real_err(A,A_m)
    % The calculation od relative error
    % Relative error is defined as the averaged
    % relative error between the estimated parameters 
    % and the true parameters 
    % i.e aij != 0 then err = err_1 or err = err_2 defined following.
	err_1 = abs(A - A_m) ./ A;
	err_2 = abs(A - A_m);
	err_1(isnan(err_1))=0;
    err_1(isinf(err_1))=0;
	err = err_1 .* double(A~=0) + err_2.*double(A==0);
	err = sum(err(:))/double(numel(A));
end