CHANGE SCALE

The minimisation method needs to calculate numerical derivatives. To estimate the descent direction, small variations are tried in each of the variable parameters. If the fit is far more sensitive to a small change in one parameter than the others, then it will only really search for the best fit for that parameter, whilst the others effectively remain constrained. To avoid this problem, each parameter has a scale size of variation. Thus, sensitive parameters have a small scale size and so are only varied by a small amount. When the parameters are input, default scale sizes are set. These should be suitable for most purposes, but sometimes better minimisation will be obtained by altering the default values. This can be seen at the start of the MINIMISATION process. The first $n+1$ fit (reduced chi-squared) values correspond to the $n$ variable parameters of the fit model. If one shows much greater variation than the others, its scale size is too large.

The CHANGE SCALE command allows the fit parameter scale sizes to be examined and values to be changed. This command itself is a sub-menu with the following commands:

EXIT: Return to FIT sub-menu

MODIFY: Make a change to one or more parameters. The number of the parameter to alter, followed by its new value.

VIEW: Display one or more parameter values, together with their name and scale size.