One error you may encounter in R is:

Error in terms.formula(formula, data = data) : invalid model formula in ExtractVars

This error occurs when you attempt to fit a decision tree in R and incorrectly specify one or more of the variables in the formula.

This tutorial shares exactly how to fix this error in practice.

**How to Reproduce the Error**

Suppose we create the following data frame in R:

**#create data frame
df <- data.frame(rating=c(88, 94, 99, 90, 76, 78, 81, 88),
points=c(14, 17, 22, 24, 25, 22, 29, 31),
assists=c(7, 7, 6, 12, 10, 11, 17, 2),
rebounds=c(7, 8, 8, 12, 9, 5, 11, 15))
#view data frame
df
rating points assists rebounds
1 88 14 7 7
2 94 17 7 8
3 99 22 6 8
4 90 24 12 12
5 76 25 10 9
6 78 22 11 5
7 81 29 17 11
8 88 31 2 15**

Now suppose we attempt to use the **rpart()** function to fit a decision tree model to the data:

**library(rpart)
#attempt to fit decision tree model to data
model <- rpart(rating ~ "points" + "assists" + "rebounds", data = df)
Error in terms.formula(formula, data = data) :
invalid model formula in ExtractVars
**

We receive an error because we used quotations around the predictor variable names, which is not allowed in the formula.

**How to Fix the Error**

The way to fix this error is to simply remove the quotations around the variable names and write the formula as follows:

**library(rpart)
#fit decision tree model
model <- rpart(rating ~ points + assists + rebounds, data = df)
#view summary of model
summary(model)
Call:
rpart(formula = rating ~ points + assists + rebounds, data = df)
n= 8
CP nsplit rel error xerror xstd
1 0.01 0 1 0 0
Node number 1: 8 observations
mean=86.75, MSE=55.1875
**

We’re able to successfully fit the model without any errors because we removed the quotations from the predictor variables in the formula.

**Additional Resources**

The following tutorials explain how to fix other common errors in R:

How to Fix: the condition has length > 1 and only the first element will be used

How to Fix: non-numeric argument to binary operator

How to Fix: dim(X) must have a positive length

How to Fix: error in select unused arguments