BACK and NOBACK

Purpose

Indicates that the estimated number of background cases is to be computed for each record after a model is fit. The NOBACK command is used to remove the %BK variable and end the automatic computation of background values.

Programs

AMFIT, GMBO/PECAN

Syntax

BACK TERM # @

NOBACK

Arguments and Subcommands

TERM #

Modifies the definition of the background rate for product additive, product additive excess, and additive models. Terms 0 through # are included in the background computations. For the product additive excess and additive models, the default background estimate includes only . For the product additive model, the default background is (that is, the default value for # is 1). For multiplicative and geometric mixture models, the background is always based on .

Remarks

The BACK command defines a variable called %BK which is used to store the estimated number of background cases. These estimates are recomputed after each model is fit. For each record the estimate is the product of the fitted rate and the person-years (AMFIT) or number of trials (GMBO).

The definition of the background rate depends on the type of model. For product additive excess, additive, and geometric mixture models, the default definition of the background rate is:

For product additive models, the default definition is:

The TERM subcommand can be used to modify the definition of the background rate for the product additive excess, product additive, and additive models. If # is specified as n, the background rate for the product additive excess risk model becomes

while the background rate for product additive models becomes

and the background rate for additive models is

The value given with the TERM subcommand does not change the definition of the background rates for geometric mixture models.

Whenever the model type is changed using the RRISK, ADD, or GMIX commands, the default background definition is restored.