The ICLIFETEST Procedure

The STRATA statement identifies the variables that determine the stratum levels. You can specify more than one variable in the STRATA statement. By default, strata are formed according to the nonmissing values of these variables. However, missing values are treated as a valid stratum level if you specify the MISSING option in the PROC ICLIFETEST statement.

The specification of several STRATA variables, such as

strata A B C;

is equivalent to the A*B*C syntax of the TABLES statement in the FREQ procedure. The number of stratum levels usually grows very rapidly along with the number of STRATA variables, so you must be cautious when specifying the list of STRATA variables.

If you specify the STRATA statement without specifying a TEST statement, the ICLIFETEST procedure produces an individual survival estimate for each stratum. On the other hand, if you specify both the STRATA and TEST statements, the ICLIFETEST procedure produces a stratified test in which the comparisons are made among the groups that are formed by the levels of the TEST variable within strata. Also, individual survival estimates are computed for all the groups that are formed by the levels of the TEST variable within a stratum.

In contrast with the STRATA statement in the LIFETEST procedure, the STRATA statement in PROC ICLIFETEST does not support the options for specifying various weight functions, stratified tests, or trend tests and to make multiple-comparison adjustments for paired differences. Instead, PROC ICLIFETEST provides these options in the TEST statement.