ciapower {Hmisc} | R Documentation

## Power of Interaction Test for Exponential Survival

### Description

Uses the method of Peterson and George to compute the power of an interaction test in a 2 x 2 setup in which all 4 distributions are exponential. This will be the same as the power of the Cox model test if assumptions hold. The test is 2-tailed. The duration of accrual is specified (constant accrual is assumed), as is the minimum follow-up time. The maximum follow-up time is then `accrual + tmin`. Treatment allocation is assumed to be 1:1.

### Usage

    ciapower(tref, n1, n2, m1c, m2c, r1, r2, accrual, tmin,
             alpha=0.05, pr=TRUE)

### Arguments

`tref` | time at which mortalities estimated
`n1` | total sample size, stratum 1
`n2` | total sample size, stratum 2
`m1c` | tref-year mortality, stratum 1 control
`m2c` | tref-year mortality, stratum 2 control
`r1` | % reduction in `m1c` by intervention, stratum 1
`r2` | % reduction in `m2c` by intervention, stratum 2
`accrual` | duration of accrual period
`tmin` | minimum follow-up time
`alpha` | type I error probability
`pr` | set to `FALSE` to suppress printing of details

### Value

power

### Side Effects

prints

### AUTHOR

Frank Harrell

Department of Biostatistics

Vanderbilt University

f.harrell@vanderbilt.edu

### References

Peterson B, George SL: Controlled Clinical Trials 14:511–522; 1993.

### See Also

`cpower`, `spower`

### Examples

    # Find the power of a race x treatment test.  25% of patients will
    # be non-white and the total sample size is 14000.
    # Accrual is for 1.5 years and minimum follow-up is 5y.
    # Reduction in 5-year mortality is 15% for whites, 0% or -5% for
    # non-whites.  5-year mortality for control subjects if assumed to
    # be 0.18 for whites, 0.23 for non-whites.
    n <- 14000
    for(nonwhite.reduction in c(0,-5)) {
      cat("\n\n\n% Reduction in 5-year mortality for non-whites:",
          nonwhite.reduction, "\n\n")
      pow <- ciapower(5,  .75*n, .25*n,  .18, .23,  15, nonwhite.reduction,
                      1.5, 5)
      cat("\n\nPower:",format(pow),"\n")
    }

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