STRATA Statement

The LIFETEST Procedure

STRATA Statement

STRATA variable < (list) > < ... variable < (list) > > < /options >;

The STRATA statement indicates which variables determine strata levels for the computations. The strata are formed according to the nonmissing values of the designated strata variables. The MISSING option can be used to allow missing values as a valid stratum level. Other options enable you to specify various k-sample tests, trend tests and stratified tests.

In the preceding syntax, variable is a variable whose values determine the stratum levels and list is a list of endpoints for a numeric variable. The values for variable can be formatted or unformatted. If the variable is a character variable, or if the variable is numeric and no list appears, then the strata are defined by the unique values of the strata variable. More than one variable can be specified in the STRATA statement, and each numeric variable can be followed by a list. Each interval contains its lower endpoint but does not contain its upper endpoint. The corresponding strata are formed by the combination of levels. If a variable is numeric and is followed by a list, then the levels for that variable correspond to the intervals defined by the list. The initial interval is assumed to start at $-\infty$ and the final interval is assumed to end at $\infty$ .

The specification of STRATA variables can have any of the following forms:

list separated by blanks		`strata age(5 10 20 30)`
list separated by commas		`strata age(5,10,20,30)`
x to y		`strata age(5 to 10)`
x to y by z		`strata age(5 to 30 by 10)`
combination of the above		`strata age(5,10 to 50 by 10)`

For example, the specification

 
   strata age(5,20 to 50 by 10) sex;

indicates the following levels for the Age variable

$\{ (-\infty,5), [5,20), [20,30), [30,40), [40,50), [50,\infty) \}$

This statement also specifies that the age strata is further subdivided by values of the variable Sex. In this example, there are 6 age groups by 2 sex groups, forming a total of 12 strata.

The specification of several variables (for example, A B C) is equivalent to the A* B*C... syntax of the TABLES statement in the FREQ procedure. The number of strata levels usually grows very rapidly with the number of STRATA variables, so you must be cautious when specifying the list of STRATA variables.

The following options can appear in the STRATA statement after a slash ("/"). Other than the MISSING option, these options are dedicated to the tests of the two or more samples of survival data.

GROUP=variable

specifies the variable whose formatted values identify the various samples whose underlying survival curves are to be compared. The tests are stratified on the levels of the STRATA variables. For instance, in a multicenter trial in which two forms of therapy are to be compared, you specify the variable identifying therapies as the GROUP= variable and the variable identifying centers as the STRATA variable, in order to perform a stratified 2-sample test to compare the therapies while controlling the effect of the centers.

MISSING

allows missing values to be a stratum level or a valid value of the GROUP= variable.

NODETAIL

suppresses the display of the rank statistics and the corresponding covariance matrices for various strata. If the TREND option is specified, the display of the scores for computing the trend tests is suppressed.

NOTEST

suppresses the k-sample tests, stratified tests, and trend tests

TREND

computes the trend tests for testing the null hypothesis that the k population hazards rate are the same versus an ordered alternatives. If there is only one STRATA variable and the variable is numeric, the unformatted values of the variable are used as the scores; otherwise, the scores are 1, 2, ... , in the given order of the strata.

TEST=(list)

enables you to select the weight functions for the k-sample tests, stratified tests, or trend tests. You can specify a list containing one or more of the following keywords.

LOGRANK: specifies the log-rank test
WILCOXON: specifies the Wilcoxon test. The test is also referred to as the Gehan test or the Breslow test.
TARONE: specifies the Tarone-Ware test
PETO: specifies the Peto-Peto test. The test is also referred to as the Peto-Peto-Prentice test.
MODPETO: specifies the modified Peto-Peto test
FLEMING( $\rho1$ , $\rho2$ ): specifies the family of tests in Harrington and Fleming (1982), where $\rho1$ and $\rho2$ are nonegative numbers. FLEMING( $\rho1$ , $\rho2$ ) reduces to the Fleming-Harrington $G^{\rho}$ family (Fleming and Harrington 1981) when $\rho2$ =0, which you can specify FLEMING( $\rho$ ) with one argument. When $\rho$ =0, the test becomes the log-rank test. When $\rho$ =1, the test should be very close to the Peto-Peto test.
LR: specifies the likelihood ratio test based on the exponential model.
ALL: specifies all the nonparametric tests with $\rho1$ =1 and $\rho2$ =0 for the FLEMING(. , .) test.

By default, TEST=(LOGRANK WILCOXON LR) for the k-sample tests, and TEST=(LOGRANK WILCOXON) for stratified and trend tests.

Previous | Next | Top of Page