Matt Bannert - 5 months ago 22

R Question

Somehow I can´t find it in my notes... nor do find the obivous on the net. How can I tell R to use a certain level as reference if I use dummy explanatories 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.

Thx in advance !

Answer

See the `relevel()`

function. Here is an example:

```
set.seed(123)
x <- rnorm(100)
DF <- data.frame(x = x,
y = 4 + (1.5*x) + rnorm(100, sd = 2),
b = gl(5, 20))
head(DF)
str(DF)
m1 <- lm(y ~ x + b, data = DF)
summary(m1)
```

Now alter the factor `b`

in `DF`

by use of the `relevel()`

function:

```
DF <- within(DF, b <- relevel(b, ref = 3))
m2 <- lm(y ~ x + b, data = DF)
summary(m2)
```

The models have estimated different reference levels.

```
> coef(m1)
(Intercept) x b2 b3 b4 b5
3.2903239 1.4358520 0.6296896 0.3698343 1.0357633 0.4666219
> coef(m2)
(Intercept) x b1 b2 b4 b5
3.66015826 1.43585196 -0.36983433 0.25985529 0.66592898 0.09678759
```