Call: FIT_FUNCTION

function

The function that should be fitted is entered as a character String. The function depends on the current set of global variables, and has X as ordinate variable. The function should only depend on global variables that are of type Number.

The global variables that are to be varied are those that appear in the argument list of the procedure call. The other globals are not changed.

reference

The Histogram is referenced via a global variable that has usually been returned by a call to BOOK_HISTOGRAM or to GET_HISTOGRAM. This argument may also be an expression that results in a histogram.

The argument is not modified on return.

x

A 1-dimensional Matrix that contains the ordinates of the points to be fitted.

The vectors x, y and err_y must have the same length, unless err_y is given as a scalar.

Not changed on exit.

y

A 1-dimensional Matrix that contains the coordinates of the points to be fitted.

The vectors x, y and err_y must have the same length, unless err_y is given as a scalar.

In the initial estimate, the data points where y\ \≤\ 0 are skipped. If there are many such points which should be taken into account for the fit, then convergence is not guaranteed.

Not changed on exit.

err_y

A 1-dimensional Matrix that contains the errors on the coordinates of the points to be fitted.

This argument can also be a scalar, which is then used as error for all points. If this argument is a vector, then it must have the same length as x and y.

Not changed on exit.

parameters

The global variables for which optimum values are to be determined have to appear in the argument list of the procedure call.

These parameters must be given a suitable initial value of type Number, for instance by using the Global statement.

On return of a successful fit, the global variables are set equal to the optimum value found. The global variables are of type Undefined if the fit fails.

errors

The errors returned are the square roots of the diagonal elements of the covariance matrix, multiplied by a correction factor equal to:

\&radic;(\&chi;\&sup2;/(#points - #parameters))

This correction factor is equal to 1 if the errors to the data points have been assigned correctly.

These arguments must be modifiable, that is, they can neither be constants nor expressions.

options

The options String may contain the keywords listed in the table below. The keywords should be separated by blanks or commata. Additional text may be added - no error message will be issued if unrecognised keywords are used.

Option	Meaning	Defaults
`EQUAL`	Assume equal weight of all points	Not default
`NOPLOT`	Don't plot the histogram and fit	Default
`NOPRINT`	Don't print details during the fit	Default
`PLOT`	Plot the histogram with fit	Not default
`POISSON`	Assume Poisson errors	Default
`PRINT`	Print details during the fit	Not default

Note:

The EQUAL and POISSON options are ignored for matrix fits since the errors on the points are explicitly provided in that case.
The plot is always made on a linear-logarithmic scale.

Go to the top level, to Call, to FIT_FUNCTION, to the topic index, to the table of contents, or to the full text.

Formatted on 21/01/18 at 16:55.