Displaying a model matrix in R

If you want to see how R will represent a factor in a model, you can use the model.matrix function.

For example, with unordered factors the model produces dummy variables.

> p = letters[1:4] |> factor()
> model.matrix(~p)
  (Intercept) pb pc pd
1           1  0  0  0
2           1  1  0  0
3           1  0  1  0
4           1  0  0  1

For ordered factors, it produces polynomial contrasts.

> p = letters[1:4] |> factor(ordered = TRUE)
> model.matrix(~p)
  (Intercept)        p.L  p.Q        p.C
1           1 -0.6708204  0.5 -0.2236068
2           1 -0.2236068 -0.5  0.6708204
3           1  0.2236068 -0.5 -0.6708204
4           1  0.6708204  0.5  0.2236068

Original source.

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Displaying a model matrix in R

Jaye Ross

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