How can I tell R to use a certain level as reference if I use binary explanatory variables in a regression?
It's just using some level by default.
lm(x ~ y + as.factor(b))
with b {0, 1, 2, 3, 4}. Let's say I want to use 3 instead of the zero that is used by R.
For those looking for a dplyr/tidyverse version. Building on Gavin Simpson solution:
# Create DF
set.seed(123)
x <- rnorm(100)
DF <- data.frame(x = x,
y = 4 + (1.5*x) + rnorm(100, sd = 2),
b = gl(5, 20))
# Change reference level
DF = DF %>% mutate(b = relevel(b, 3))
m2 <- lm(y ~ x + b, data = DF)
summary(m2)