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#### 3.6.4.1 control variables

The default epsilon limit (1e-5) may be changed by declaring a value for

FIT_LIMIT

When the sum of squared residuals changes between two iteration steps by a factor less than this number (epsilon), the fit is considered to have ’converged’.

The maximum number of iterations may be limited by declaring a value for

FIT_MAXITER

A value of 0 (or not defining it at all) means that there is no limit.

If you need even more control about the algorithm, and know the Marquardt-Levenberg algorithm well, there are some more variables to influence it. The startup value of ‘lambda‘ is normally calculated automatically from the ML-matrix, but if you want to, you may provide your own one with

FIT_START_LAMBDA

Specifying FIT_START_LAMBDA as zero or less will re-enable the automatic selection. The variable

FIT_LAMBDA_FACTOR

gives the factor by which ‘lambda‘ is increased or decreased whenever the chi-squared target function increased or decreased significantly. Setting FIT_LAMBDA_FACTOR to zero re-enables the default factor of 10.0.

Other variables with the FIT_ prefix may be added to fit, so it is safer not to use that prefix for user-defined variables.

The variables FIT_SKIP and FIT_INDEX were used by earlier releases of ‘gnuplot‘ with a ’fit’ patch called ‘gnufit‘ and are no longer available. The datafile every modifier provides the functionality of FIT_SKIP. FIT_INDEX was used for multi-branch fitting, but multi-branch fitting of one independent variable is now done as a pseudo-3D fit in which the second independent variable and using are used to specify the branch. See multi-branch.

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