Plotting power vs. effect size using R pwr package - r

Question

I can successfully create plots of power vs. sample size in R using the pwr package. Example code below.
library(pwr)
library(tidyverse)
plot.out <- pwr.t2n.test(n1=30, n2=30, d=0.5, alternative="two.sided")
#See output in link below
plot(plot.out)
plot() output
I would like to create a similar plot -- a two-sample t-test in which effect size is on the y-axis and power is on the x-axis, with fixed sample sizes.
Is there a way to do this using pwr and/or the plot function? Or would I have to unlist the plot.out object and use it somehow?
I'm still new to power curves in R. Thanks in advance for any advice.

Answer 1

In the code below the power is computed in a loop on effect size d_seq. Then the power d is extracted from the results list, a data.frame is created and plotted.
library(pwr)
library(ggplot2)
d_seq <- seq(0, 2, by = 0.1)
pwr_list <- lapply(d_seq, function(d){
pwr.t2n.test(n1 = 30, n2 = 30,
d = d,
power = NULL,
sig.level = 0.05,
alternative = "two.sided")
})
pwr <- sapply(pwr_list, '[[', 'power')
dfpwr <- data.frame(power = pwr, effect.size = d_seq)
ggplot(dfpwr, aes(effect.size, power)) +
geom_point(size = 2, colour = "black") +
geom_line(size = 0.5, colour = "red") +
scale_y_continuous(labels = scales::percent) +
xlab("effect size") +
ylab(expression("test power =" ~ 1 - beta))
To draw a line where power is 80% and get the effect size, first compute the effect size from the pwr vector by linear interpolation.
pwr80 <- approx(x = pwr, y = d_seq, xout = 0.8)
Now create a label for geom_text and plot it.
lbl80 <- paste("Power = 80%\n")
lbl80 <- paste(lbl80, "Effect size =", round(pwr80$y, 2))
ggplot(dfpwr, aes(effect.size, power)) +
geom_point(size = 2, colour = "black") +
geom_line(size = 0.5, colour = "red") +
geom_hline(yintercept = 0.8, linetype = "dotted") +
geom_text(x = pwr80$y, y = pwr80$x,
label = lbl80,
hjust = 1, vjust = -1) +
scale_y_continuous(labels = scales::percent) +
xlab("effect size") +
ylab(expression("test power =" ~ 1 - beta))
To also draw a vertical line, add
geom_vline(xintercept = pwr80$y, linetype = "dotted")