Power

# Load 'pwr' package
library(pwr)

# 80% power

# Small effect size
small80p <- pwr.t.test(sig.level = 0.05, d = 0.2, 
                       power = 0.8, type="two.sample", alternative="two.sided") 
small80p  

# Medium effect size
med80p <- pwr.t.test(sig.level = 0.05, d = 0.5, 
                     power = 0.8, type="two.sample", alternative="two.sided") 
med80p  

# Large effect size
large80p <- pwr.t.test(sig.level = 0.05, d = 0.8, 
                       power = 0.8, type="two.sample", alternative="two.sided") 
large80p