Purpose
Indicates that a model is to be fit and, optionally, specifies or modifies covariates in the default subterm and specifies BY variables.
Syntax
FIT {model formula}{; BY catnames} @
Arguments and Subcommands
model formula
Variables and operators indicating how the default subterm is to be modified. (The format of the model formula is described in the introductory remarks at the beginning of Model Specification Commands.) If the model formula is omitted, the default term is not changed.
BY catnames
One or more categorical variables. Separate models will be fit for each level of the BY variables.
Remarks
Unless explicitly modified by the user, the initial estimates used in a fit are the final estimates for the parameters from the most recent model. Thus, where convergence has not been obtained after the maximum number of iterations, one need only reissue the FIT command without any arguments to continue the iterations.
There are a number of options related to aspects of the fitting algorithm, the nature of the output produced for a fitted model, and the interpretation of the optional FIT command model formula which may be set using the FITOPT command.
A model formula given with a FIT command will affect the current default subterm. Initially, the loglinear subterm of term 0 is the default subterm; however, this can be changed by the user with the FITOPT command.
Variables listed with the BY subcommand must have been previously defined by a LEVELS command. Time-dependent variables defined in TTRAN code cannot be listed as BY variables.
Examples
a) To fit the currently specified model without any modification:
FIT @
b) To add sex to the current default subterm and fit the model:
FIT + sex @
c) To replace the current default subterm with a term which includes %CON as well as parameters associated with sex, lage, and lage2:
FIT %CON:-1 sex lage lage2=0 @
%CON is initialized to -1 while the parameter associated with lage2 is fixed at 0.
d) To fit the previous model separately for males and females:
LEVELS sex @
FIT %CON:-1 lage lage2=0; BY sex @