# ciapower: Power of Interaction Test for Exponential Survival In harrelfe/Hmisc: Harrell Miscellaneous

## 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

 ```1 2``` ```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

power

prints

## AUTHOR

Frank Harrell

Department of Biostatistics

Vanderbilt University

## References

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

`cpower`, `spower`
 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14``` ```# 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") } ```