STRATA Statement

The LIFETEST Procedure

STRATA Statement

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

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.

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 STRATA statement 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 STRATA list.

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