function [mle, info, n] = varhetsec(y, x, mu0, mu1, TOL, Nmax)
% *************************************************************************
%
% FILE: varhetsec.m
%
% Date Created: Feb 10, 2004
% Author: Mark E. Irwin
%
% Contents: Finds MLE and information for variance heterogeneity model 
%           Y ~ N(mu, (mu*x)^2) by Secant method.  The information is
%           based on a numerical approximation to the derivative.
%
% Revision History
% Date          Name            Changes/Reasons
%
% *************************************************************************
%
% Input:
%
%   y: response vector
%   x: variance covariate vector
%   mu0: initial guess for MLE
%   mu1: second guess for MLE
%   TOL: convergence tolerance
%   Nmax: number of functional iterations
%
% Output:
%
%   mle: mle
%   info: information
%   n: number of iterations until convergence
%
% *************************************************************************

% Initialize loop

i = 0;
diff = 2 * TOL;  % guarantees at least one pass through loop

while (i < Nmax && diff > TOL)
  i = i + 1;
  f0 = hetscore(y, x, mu0);
  f1 = hetscore(y, x, mu1);
  deriv = (f1 - f0)/(mu1 - mu0);
  mui = mu1 - f1/deriv;
  diff = abs(mu1 - mui);
  mu0 = mu1;
  mu1 = mui;
end

if (diff > TOL)
  warning('Method did not converage after Nmax steps');
end

mle = mui;
info = -deriv;
n = i;