Re: lsqcurvefit-converge?

Thanks for dealing with my problem,

>
>> I have a problem considering lsqcurvefit as a routine to fit a
> number
>> of data. More specifically, the analyitical expression that is
>> supposed to be used to fit the data I have has 4 parameters.
> Let's
>> say that
>>
>> beta=[K;D_2;w;M]; (where I set the values)
>> LB=[0;0;0;0];
>> UB=[10*K;3;3;1];
>>
>> then I ran the lscurvefit to compute the parameters ad I get :
> sum of
>> residuals =0.016. and the warning messag
>
> Are you truly looking at the sum of the residuals?
> Or did you mean the sum of squares of the residuals?
> The latter is what lsqcurvefit uses.
>

Yes, you are right I was referring to the sum of sqares of the
residuals.

> Note that it is quite easy to pose a problem where
> the simple sum of residuals is not at all well posed.
>
>
>
>> e 'Optimization terminated successfully:
>> Relative function value changing by less than OPTIONS.TolFun'
>>
>> The value for D_2=0.57
>> Then, I change the LB and UB to
>> LB=[0;0;0;0];
>> UB=[10*K;30;30;1];
>> and if I run the same routine then, I get exatly the same
values
> for
>> all parameters except from D_2 and w (the parameters whose LB
and
> UB
>> have been changed)
>> , the same warning message, exit flag1 as previously, same sum
of
>> residuals. Do you think that this sounds reasonable?
>
> If the objective (i.e., the sum of SQUARES of the
> residuals) is the same, then your problem may be
> indeterminate. That is, any solution along some
> path through your parameter space may be yielding
> the same minimum objective. In that event it is
> rather arbitrary which "solution" from this infinite
> set lsqcurvefit finds. Simply changing something
> trivial like a non-active bound may result in a
> different final solution.
>
>

Then, does it mean that the value of the parameter D_2, for instance,
should lie in the range [D_2-A,D_2+A] where A is something like an
error in the estimate of the value and the inability to set a clear
value for the upper and lower limits of the rest of the parameters
result in this fluctuation? In this case, evenb if I get an EXIT=1
message, should I accept the value? Is there another function I could
use to check the results?

Thanks a lot for the assistance

Jonn
.

Relevant Pages

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