using converged estimations as starting values, why it still iterates?

I am estimating probit model and some other nonlinear models with Gauss. After it converges at 1e-5 tolerance using optmum or Qnewton to do the optimization, I plug in the estimated coefficients as the starting value. I expect that there will be no iteration since it's already converged. But it iterates again. And the gradient of the first iteration sometimes is even larger than 1e-2.  Why is it?

3 Answers



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How many places do you save from converged values?  The solution is for 16 decimal places and if your start values are less than that, 5 places, say, then it will have to iterate for those remaining 11 places.



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Thank you! I saved 8 decimal places, and the tolerance is 1e-5. But when it iterates again, some of the gradients are even greater than 1e-4. When it converges again at 1e-5, some of the estimates are different than the starting value (or previous estimates) at 2nd decimal.

Maybe I have a really flat objective function?



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You are likely right about the flat function.  Check the condition of your Hessian.   I'm not sure what Application you're using, but if it's CMLMT,

struct cmlmtResults out;

out = CMLmt(&lpr,p0,d0,c0);

print log10(cond(out.hessian));

The number printed is the approximate number of places lost in computing the inverse for the covariance matrix of the parameters.  If it's greater than 8, it is ill-conditioned indicating a relatively flat function.  If it's greater than 16, it is a not positive definite matrix.

 

 

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