# ciapower

##### Power of Interaction Test for Exponential Survival

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

##### References

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

##### See Also

##### Examples

```
# NOT RUN {
# 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")
}
# }
```

*Documentation reproduced from package Hmisc, version 4.3-1, License: GPL (>= 2)*