Example of using an offset in a linear model

OffsetInLinearModel.R

# offsets
# Simulate some data for this example
set.seed(12)
sig <- matrix(c(1.1, 1, 1, 1.1), 2, 2)
df1 <- data.frame(MASS::mvrnorm(n = 100, c(30, 30), Sigma = sig))
names(df1) <- c("x", "y")

# Plot the data, and a 1:1 line
plot(df1$x, df1$y)
abline(0, 1)

# Ordinary model with no offset.
# Slope summary gives statistics for difference from 0.
mod1 <- lm(y ~ x, data = df1)
summary(mod1)

# Model WITH an offset of x.
# Slope summary now gives statistics for difference from 1.
mod2 <- lm(y ~ x + offset(x), data = df1)
summary(mod2)