# How to Fix in R: invalid model formula in ExtractVars

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.