Part4
Step-by-Step Guide to Assignment 8.4
Problem 4. Kaplan-Meier Survival Analysis (With Strata)
a. Run a Kaplan-Meier analysis in SPSS, using Time as the Time variable and Event as the Status variable (Be sure to define the event). Add Interval as the Strata variable.
Step 1. Analyze ( Survival ( Kaplan Meier.
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Step 2. Remove the Treatment Status [tx] variable from the Factor box Select The order of the time interval [interval] and place it in the Strata box.
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Step 3. Click Options. In statistics, select Survival table(s) and Mean and median survival. Select Survival in plots. Click continue. Click OK.
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SPSS Output:
b. Produce a plot of the survival function for each strata.
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c. Why would you want to produce Kaplan-Meier Survival Curves after stratifying for a variable?
Stratification allows you to compare survival by different levels of a variable, adding yet another layer of adjustment (control) in a model. In this example, total time on study was subdivided into 5 time intervals. Because the probability of survival changes over time, stratification enabled survival patterns to be compared at various stages or observation times. It may be that despite treatment, length of time lived beyond diagnosis has an affect on survival. Stratification controls for this effect.