I want to test whether coefficients in one linear regression are different from each other or whether at least one of them is significantly different from one certain value, say 0, this seems quite intuitive to do in Stata. For example
webuse iris
reg iris seplen sepwid petlen
seplen==sepwid==petlen
seplen==sepwid==petlen==0
I wonder how I can do this if I want to test this in R?
The car
package has a simple function to do that.
First, fit your model:
model <- lm(Sepal.Length ~ Sepal.Width + Petal.Length, data = iris)
Than you may test different linear hypothesis using the linearHypothesis
function, for instance:
library(car)
# tests if the coefficient of Sepal.Width = Petal.Length
linearHypothesis(model, "Sepal.Width = Petal.Length")
Linear hypothesis test
Hypothesis:
Sepal.Width - Petal.Length = 0
Model 1: restricted model
Model 2: Sepal.Length ~ Sepal.Width + Petal.Length
Res.Df RSS Df Sum of Sq F Pr(>F)
1 148 16.744
2 147 16.329 1 0.4157 3.7423 0.05497 .
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
# Other examples:
# tests if the coefficient of Sepal.Width = 0
linearHypothesis(model, "Sepal.Width = 0")
# tests if the coefficient of Sepal.Width = 0.5
linearHypothesis(model, "Sepal.Width = 0.5")
# tests if both are equal to zero
linearHypothesis(model, c("Sepal.Width = 0", "Petal.Length = 0"))