# Non-Linear Regression in R with Decision Trees

While linear regression models have been widely used in the field of statistics for many years, there are numerous situations where the relationship between predictors and the output variable is non-linear and complex. In such cases, non-linear regression models, such as decision trees, come into play. In the R programming language, there are many packages available that provide functionalities to build such non-linear regression models.

This article will provide an extensive overview of non-linear regression using decision trees in R, discuss their practical implementation, and illustrate with examples.

### Understanding Non-Linear Regression

In statistics, non-linear regression is a form of regression analysis in which observational data are modeled by a function which is a non-linear combination of the model parameters and depends on one or more independent variables. The data is fitted by a method of successive approximations.

### Decision Trees for Regression

Decision trees are not just for classification problems; they can also be used for regression. Regression trees are used when the response variable is numeric or continuous. In a regression tree, the value of the target variable for each leaf node is the mean value of the target variable of the training observations contained in that leaf.

Regression trees offer a nice feature of interpretability: the decision rules are easily visualizable and thus the predictions are easy to explain. Moreover, decision trees can handle non-linear relationships well, providing a useful tool for non-linear regression.

### Using Decision Trees for Non-Linear Regression in R

To illustrate non-linear regression using decision trees in R, we will use the rpart package, which stands for Recursive Partitioning and Regression Trees. The rpart function in this package can handle both classification and regression trees.

First, we will install and load the rpart package.

# install the package
install.packages('rpart')

# load the package
library(rpart)

Now, let’s assume we have a dataset df with predictor variables ‘var1’, ‘var2’, ‘var3’, and our target variable ‘target’. Here’s how we can use the rpart function to create a regression tree.

# fit the model
fit <- rpart(target ~ var1 + var2 + var3, data = df, method = "anova")

# print the model
print(fit)

# plot tree
plot(fit, uniform = TRUE, main = "Regression Tree")
text(fit, use.n = TRUE, all = TRUE, cex = .8)

In this example, we use the rpart() function to fit a regression tree model. The argument method = "anova" is used to specify that we want to perform regression (as opposed to classification, for which we would use method = "class").

The plot() function is used to visualize the tree. The argument uniform = TRUE ensures that all splits that look the same are drawn at the same depth, while main provides a title for the plot. The text() function is used to label the tree’s nodes.

### Evaluating the Model

Like any machine learning model, it’s crucial to evaluate the performance of a decision tree regression model. One common metric for regression problems is Root Mean Squared Error (RMSE). The caret package in R provides the RMSE() function to compute this.

# install and load the caret package
install.packages('caret')
library(caret)

# make predictions
predictions <- predict(fit, newdata = df)

# compute RMSE
rmse <- RMSE(predictions, df\$target)
print(rmse)

In this code, we first install and load the caret package. Then we use the predict() function to generate predictions using our decision tree model, and finally compute the RMSE using RMSE().

### Conclusion

Decision trees provide a powerful and interpretable method for non-linear regression. They are capable of capturing complex patterns in the data and can be easily visualized, making them a valuable tool in the machine learning toolkit. R, with its wide range of packages and functions, is an excellent platform for implementing decision trees for non-linear regression.

However, as with any model, it’s important to remember that decision trees have their limitations – they can be sensitive to small changes in the data and are prone to overfitting, especially when dealing with complex datasets. It’s often beneficial to use ensemble methods like random forests or gradient boosting to overcome these limitations. But with careful feature engineering, parameter tuning, and model validation, decision trees can be an effective tool for non-linear regression tasks.