| power.prop.test {stats} | R Documentation |
Power Calculations for Two-Sample Test for Proportions
Description
Compute the power of the two-sample test for proportions, or determine parameters to obtain a target power.
Usage
power.prop.test(n = NULL, p1 = NULL, p2 = NULL, sig.level = 0.05,
power = NULL,
alternative = c("two.sided", "one.sided"),
strict = FALSE, tol = .Machine$double.eps^0.25)
Arguments
n |
number of observations (per group) |
p1 |
probability in one group |
p2 |
probability in other group |
sig.level |
significance level (Type I error probability) |
power |
power of test (1 minus Type II error probability) |
alternative |
one- or two-sided test. Can be abbreviated. |
strict |
use strict interpretation in two-sided case |
tol |
numerical tolerance used in root finding, the default providing (at least) four significant digits. |
Details
Exactly one of the parameters n, p1, p2,
power, and sig.level must be passed as NULL, and that
parameter is determined from the others. Notice that sig.level
has a non-NULL default so NULL must be explicitly passed if you
want it computed.
If strict = TRUE is used, the power will include the probability of
rejection in the opposite direction of the true effect, in the two-sided
case. Without this the power will be half the significance level if the
true difference is zero.
Note that not all conditions can be satisfied, e.g., for
power.prop.test(n=30, p1=0.90, p2=NULL, power=0.8, strict=TRUE)
there is no proportion p2 between p1 = 0.9 and 1, as
you'd need a sample size of at least n = 74 to yield the
desired power for (p1,p2) = (0.9, 1).
For these impossible conditions, currently a warning
(warning) is signalled which may become an error
(stop) in the future.
Value
Object of class "power.htest", a list of the arguments
(including the computed one) augmented with method and
note elements.
Note
uniroot is used to solve power equation for unknowns, so
you may see errors from it, notably about inability to bracket the
root when invalid arguments are given. If one of p1 and
p2 is computed, then p1 < p2 is assumed and will hold,
but if you specify both, p2 \le p1 is allowed.
Author(s)
Peter Dalgaard. Based on previous work by Claus Ekstrøm
See Also
Examples
power.prop.test(n = 50, p1 = .50, p2 = .75) ## => power = 0.740
power.prop.test(p1 = .50, p2 = .75, power = .90) ## => n = 76.7
power.prop.test(n = 50, p1 = .5, power = .90) ## => p2 = 0.8026
power.prop.test(n = 50, p1 = .5, p2 = 0.9, power = .90, sig.level=NULL)
## => sig.l = 0.00131
power.prop.test(p1 = .5, p2 = 0.501, sig.level=.001, power=0.90)
## => n = 10451937
try(
power.prop.test(n=30, p1=0.90, p2=NULL, power=0.8)
) # a warning (which may become an error)
## Reason:
power.prop.test( p1=0.90, p2= 1.0, power=0.8) ##-> n = 73.37