The TWOSAMPLESURVIVAL statement performs power and sample size analyses for comparing two survival curves. The logrank, Gehan, and TaroneWare rank tests are supported.
Table 77.23 summarizes the options available in the TWOSAMPLESURVIVAL statement.
Table 77.23: TWOSAMPLESURVIVAL Statement Options
Option 
Description 

Define analysis 

Specifies the statistical analysis 

Specify analysis information 

Specifies the accrual time 

Specifies the significance level 

Specifies the followup time 

Specifies the number of sides and the direction of the statistical test or confidence interval 

Specifies the total time 

Specify effects 

Defines a survival curve 

Specifies the median survival times in each group 

Specifies exponential hazard rates of the survival curve for each group 

Specifies the survival curve for each group 

Specifies the hazard ratio 

Specifies the exponential hazard rate of the survival curve for the reference group 

Specifies the survival curve for the reference group 

Specify loss information 

Specifies the exponential loss survival curve for each group 

Specifies the exponential hazards of the loss in each group 

Specifies the median times of the loss in each group 

Specify sample size and allocation 

Specifies the common accrual rate per group 

Specifies the total accrual rate 

Specifies the expected total number of events 

Specifies the accrual rate for each group 

Specifies the two group sample sizes 

Specifies the sample size allocation weights for the two groups 

Enables fractional input and output for sample sizes 

Specifies the common sample size per group 

Specifies the sample size 

Specify power 

Specifies the desired power of the test 

Specify computational method 

Specifies the number of subintervals per unit time 

Control ordering in output 

Controls the output order of parameters 
Table 77.24 summarizes the valid result parameters for different analyses in the TWOSAMPLESURVIVAL statement.
Table 77.24: Summary of Result Parameters in the TWOSAMPLESURVIVAL Statement
To specify the survival curves, choose one of the following parameterizations:
arbitrary piecewise linear or exponential curves (by using the CURVE= and GROUPSURVIVAL= options)
curves with proportional hazards (by using the CURVE= , REFSURVIVAL= , and HAZARDRATIO= options)
exponential curves, by using one of the following parameterizations:
median survival times (by using the GROUPMEDSURVTIMES= option)
the hazard ratio and the hazard of the reference curve (by using the HAZARDRATIO= and REFSURVEXPHAZARD= options)
the individual hazards (by using the GROUPSURVEXPHAZARDS= option)
To specify the study time, use any two of the following three options:
accrual time (by using the ACCRUALTIME= option)
followup time (by using the FOLLOWUPTIME= option)
total time, the sum of accrual and followup times (by using the TOTALTIME= option)
To specify the sample size and allocation, choose one of the following parameterizations:
sample size per group in a balanced design (by using the NPERGROUP= option)
accrual rate per group in a balanced design (by using the ACCRUALRATEPERGROUP= option)
total sample size and allocation weights (by using the NTOTAL= and GROUPWEIGHTS= options)
total accrual rate and allocation weights (by using the ACCRUALRATETOTAL= and GROUPWEIGHTS= options)
expected total number of events and allocation weights (by using the EVENTSTOTAL= and GROUPWEIGHTS= options)
individual group sample sizes (by using the GROUPNS= option)
individual group accrual rates (by using the GROUPACCRUALRATES= option)
The values of parameters that involve expected number of events or accrual rate are converted internally to the analogous sample size parameterization (that is, the NPERGROUP= , NTOTAL= , or GROUPNS= option) for the purpose of sample size adjustments according to the presence or absence of the NFRACTIONAL option.
To specify the exponential loss curves, choose one of the following parameterizations:
a point on the loss curve of each group (by using the CURVE= and GROUPLOSS= options)
median loss times (by using the GROUPMEDLOSSTIMES= option)
the individual loss hazards (by using the GROUPLOSSEXPHAZARDS= option)
This section summarizes the syntax for the common analyses supported in the TWOSAMPLESURVIVAL statement.
You can use the NPERGROUP= option in a balanced design and specify piecewise linear or exponential survival curves by using the CURVE= and GROUPSURVIVAL= options, as in the following statements. Default values for the SIDES= , ALPHA= , NSUBINTERVAL= , and GROUPLOSSEXPHAZARDS= options specify a twosided test with a significance level of 0.05, an assumption of no loss to followup, and the use of 12 subintervals per unit time in computations.
proc power; twosamplesurvival test=logrank curve("Control") = (1 2 3):(0.8 0.7 0.6) curve("Treatment") = (5):(.6) groupsurvival = "Control"  "Treatment" accrualtime = 2 followuptime = 1 npergroup = 50 power = .; run;
In the preceding example, the "Control" curve is piecewise linear (since it has more than one point), and the "Treatment" curve is exponential (since it has only one point).
You can also specify an unbalanced design by using the NTOTAL= and GROUPWEIGHTS= options and specify piecewise linear or exponential survival curves with proportional hazards by using the CURVE= , REFSURVIVAL= , and HAZARDRATIO= options:
proc power; twosamplesurvival test=logrank curve("Control") = (1 2 3):(0.8 0.7 0.6) refsurvival = "Control" hazardratio = 1.5 accrualtime = 2 followuptime = 1 groupweights = (1 2) ntotal = . power = 0.8; run;
Instead of computing sample size, you can compute the accrual rate by using the ACCRUALRATETOTAL= option:
proc power; twosamplesurvival test=logrank curve("Control") = (1 2 3):(0.8 0.7 0.6) refsurvival = "Control" hazardratio = 1.5 accrualtime = 2 followuptime = 1 groupweights = (1 2) accrualratetotal = . power = 0.8; run;
or the expected number of events by using the EVENTSTOTAL= option:
proc power; twosamplesurvival test=logrank curve("Control") = (1 2 3):(0.8 0.7 0.6) refsurvival = "Control" hazardratio = 1.5 accrualtime = 2 followuptime = 1 groupweights = (1 2) eventstotal = . power = 0.8; run;
You can also specify sample sizes with the GROUPNS= option and specify exponential survival curves in terms of median survival times:
proc power; twosamplesurvival test=logrank groupmedsurvtimes = (16 22) accrualtime = 6 totaltime = 18 groupns = 40  60 power = .; run;
You can also specify exponential survival curves in terms of the hazard ratio and reference hazard. The default value of the GROUPWEIGHTS= option specifies a balanced design.
proc power; twosamplesurvival test=logrank hazardratio = 1.2 refsurvexphazard = 0.7 accrualtime = 2 totaltime = 4 ntotal = 100 power = .; run;
You can also specify exponential survival curves in terms of the individual hazards, as in the following statements:
proc power; twosamplesurvival test=logrank groupsurvexphazards = 0.7  0.84 accrualtime = 2 totaltime = 4 ntotal = . power = 0.9; run;
In addition to the logrank test, you can also specify the Gehan tank test, as in the following statements. Default values for the SIDES= , ALPHA= , NSUBINTERVAL= , and GROUPLOSSEXPHAZARDS= options specify a twosided test with a significance level of 0.05, an assumption of no loss to followup, and the use of 12 subintervals per unit time in computations.
proc power; twosamplesurvival test=gehan groupmedsurvtimes = 5  7 accrualtime = 3 totaltime = 6 npergroup = . power = 0.8; run;
You can also specify the TaroneWare tank test, as in the following statements. Default values for the SIDES= , ALPHA= , NSUBINTERVAL= , and GROUPLOSSEXPHAZARDS= options specify a twosided test with a significance level of 0.05, an assumption of no loss to followup, and the use of 12 subintervals per unit time in computations.
proc power; twosamplesurvival test=taroneware groupmedsurvtimes = 5  7 accrualtime = 3 totaltime = 6 npergroup = 100 power = .; run;