power for one-sided vs two-sided tests

power_one-sided_two-sided.R

# Code by Chelsea Parlett, small additions by Daniel Lakens
library(pwr)
library(ggplot2)

#set up vector of effect sizes
es <- seq(0.01,1, length = 1000)
#specify power for test 
pow <- 0.8

#calculate ns (sample size for 80% power in two-sided test)
ns <- sapply(es, function(x) pwr.t.test(d = x,
                                        power = pow,
                                        sig.level = 0.05,
                                        type = "one.sample",
                                        alternative = "two.sided")$n)

#calculate ns (sample size for 80% power in one-sided test)
ns1 <- sapply(es, function(x) pwr.t.test(d = x,
                                         power = pow,
                                         sig.level = 0.05,
                                         type = "one.sample",
                                         alternative = "greater")$n)

onevstwopower <- data.frame(effect_size = c(es,es), n = c(ns,ns1),
                            type = c(rep("two",length(ns)),
                                     rep("one", length(ns1))))

ggplot(onevstwopower, aes(x = effect_size, y = n, color = type)) +
  geom_line() + theme_minimal()

#calculate ratio - ranges from 0.788 to 0.776
ratio <- ns1/ns
ratio