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Chapter 12
Note: Ifyour segmented, time-dependent predictor is constant within segments, as in the blood
pressure example given above, it may be easier for you to specify the piecewise-constant,
time-dependent predictorby splitting subjects across multiple cases. See the discussion on
Subject Identifiersi n ComplexSamples Cox Regression on p. 74 for moreinformation.
In the Define Time-Dependent Predictor dialog box, you can use the function-building controls
to build the expression for the time-dependent covariate, or you can enterit directly in the
Numeric Expression text area. Note that string constants must be enclosed in quotation marks
or apostrophes, and numeric constants must be typed in American format, with the dot as the
decimal delimiter. The resulting variable is given the nameyou specify and should be included
as a factor or covariate on the Predictorstab.
Subgroups
Figure 12-5
Cox Regression dialog box, Subgroups tab
BaselineStrata . A separate baseline hazard and survival function is computed for each value of
this variable, whilea single set of model coefficients is estimated across strata.
SubpopulationVariable. Specify a variable to definea subpopulation. The analysis is performed
only for the selected category ofthe subpopulation variable.