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Fixing aliased coefficients in a model in R involves using the lm.influence() function to identify and remove the aliased coefficients. The lm.influence() function will return a list of aliased coefficients that need to be removed from the model, which can be done by using the update() function to update the existing model with the coefficients removed. This process will help ensure that the model is more accurate and reliable.
One error you may encounter in R is:
Error in vif.default(model) : there are aliased coefficients in the model
This error typically occurs when exists in a regression model. That is, two or more predictor variables in the model are highly (or perfectly) correlated.
When this occurs, we say that one variable is an ‘alias’ of another variable, which causes problems when fitting a regression model.
The following example shows how to fix this error in practice.
How to Reproduce the Error
Suppose we fit the following in R:
#make this example reproducible
set.seed(0)
#define data
x1 <- rnorm(100)
x2 <- rnorm(100)
x3 <- x2*3
y <- rnorm(100)
#fit regression model
model <- lm(y~x1+x2+x3)
We can use the vif() function from the car package to calculate the VIF values for each predictor variable in the model to determine if multicollinearity is a problem:
library(car)
#calculate VIF values for predictor variables
vif(model)
Error in vif.default(model) : there are aliased coefficients in the model
We receive an error that “there are aliased coefficients in the model.“
This tells us that two or more predictor variables in the model are perfectly correlated.
How to Fix the Error
To determine which predictor variables are perfectly correlated, we can use the cor() function to create a for the variables:
#place variables in data frame
df <- data.frame(x1, x2, x3, y)
#create correlation matrix for data frame
cor(df)
x1 x2 x3 y
x1 1.00000000 0.126886263 0.126886263 0.065047543
x2 0.12688626 1.000000000 1.000000000 -0.009107573
x3 0.12688626 1.000000000 1.000000000 -0.009107573
y 0.06504754 -0.009107573 -0.009107573 1.000000000
We can see that the variables x2 and x3 have a of 1. This tells us that these two variables are causing the error because they’re perfectly correlated.
To fix this error, we simply need to fit the regression model again and leave out one of these two variables.
For simplicity, let’s remove x3 and fit the regression model again:
library(car)
#make this example reproducible
set.seed(0)
#define data
x1 <- rnorm(100)
x2 <- rnorm(100)
x3 <- x2*3
y <- rnorm(100)
#fit regression model
model <- lm(y~x1+x2)
#calculate VIF values for predictor variables in model
vif(model)
x1 x2
1.016364 1.016364
Note that we don’t receive any error this time when calculating the VIF values for the model because multicollinearity is no longer an issue.
Related:
The following tutorials explain how to fix other common errors in R: