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SPSS® 12.0 Command Syntax Reference

SPSS® 12.0 Command Syntax Reference

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KM 789<br />

• Only one status variable can be listed on the STATUS subcommand. You must specify the<br />

value(s) indicating that the event occurred.<br />

• Only one variable can be specified on the STRATA subcommand. If you have more than<br />

one stratum, use the transformation language to create a single variable to specify on the<br />

STRATA subcommand.<br />

Operations<br />

• KM deletes all cases that have negative values for the survival variable.<br />

• KM estimates the survival function and associated statistics for each combination of factor<br />

and stratum.<br />

• Three statistics can be computed to test the equality of survival functions across factor<br />

levels within a stratum or across all factor levels while controlling for strata. The statistics<br />

are the log rank (Mantel-Cox), the generalized Wilcoxon (Breslow), and the Tarone-<br />

Ware tests.<br />

• When the PLOTS subcommand is specified, KM produces one plot of survival functions<br />

for each stratum, with all factor levels represented by different symbols or colors.<br />

Limitations<br />

Example<br />

• Maximum 35 factor levels (symbols) can appear in a plot.<br />

KM LENGTH BY SEXRACE<br />

/STATUS=EMPLOY EVENT (1) LOST (2)<br />

/STRATA=LOCATION.<br />

• Survival analysis is used to examine the length of unemployment. The survival variable<br />

LENGTH contains the number of months a subject is unemployed. The factor variable<br />

SEXRACE combines sex and race factors.<br />

• A value of 1 on the variable EMPLOY indicates the occurrence of the event (employment).<br />

All other observed cases are censored. A value of 2 on EMPLOY indicates cases lost to<br />

follow-up. Cases with other values for EMPLOY are known to have remained unemployed<br />

during the course of the study. KM separates the two types of censored cases in the KM<br />

table if LOST is specified.<br />

• For each combination of SEXRACE and LOCATION, one KM table is produced, followed<br />

by the mean and median survival time with standard errors and confidence intervals.<br />

Survival and Factor Variables<br />

You must identify the survival and factor variables for the analysis.<br />

• The minimum specification is one, and only one, survival variable.<br />

• Only one factor variable can be specified using the BY keyword. If you have more than one<br />

factor, create a new variable combining all factors. There is no limit to the factor levels.

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