## Simple Regression Trees in Julia

Being a data analyst, it’s a bit embarrassing how little experience I have with the new hotness of machine learning. I recently had a conversation with an individual who mentioned that they often employ decision and regression trees as a data exploration method and this prompted me to start looking into them.

Decision and regression trees are an awesome tool because of how transparent the end result is. It’s easy to understand and to explain to others who might be weary of implementing something opaque. In the simplest trees, they ask a series of yes-no questions such as: is a certain variable greater than some number. With each question you progress through different paths until you reach a terminal node. This terminal node will give you a prediction, either a classification or a value, depending on the type of tree. This process is extremely easy to follow and is the biggest selling point of decision trees.

Another advantage of decision trees is the simplicity of the algorithm used to create the tree. There are 3 basic steps that go into creating a tree. The first is a calculation on some cost function that we want to minimize. In the case of regression trees the cost function is usually just the mean squared error of all observations at that particular node. Secondly, each variable is iterated over to find the optimal way to divide the observations into two groups. Optimal, in this case, refers to the smallest mean squared error. And finally, once the optimal division is found the process is repeated on the two subgroups. This continues until certain predefined conditions are reached like minimum number of observations at a node.

In fact, the algorithm is so simple I decided to implement a basic regression tree in Julia as a learning exercise. Julia is an awesome statistical computing language thats main advantage is speed. Code written in Julia is often several times faster than the equivalent R or Python code for non-trivial calculations. My implementation is rather limited compared to the ‘rpart’ package in R or even the ‘DecisionTrees.jl’ package available in Julia. The idea was to gain a better understanding of how decision trees actually work and not to replace any of the already great implementations available.

I tested my implementation on the 'cu.summary' dataset from 'rpart'. This dataset contains information on a small number of cars and regressing on mileage gives the following tree:
```Price < 9415.84 : 1
Price < 6696.9 : 2
4 : 34.0 : 3
7 : 30.714285714285715 : 3
Type IN String["Small","Sporty","Compact"] : 2
Price < 11475.8 : 3
Reliability IN String["average","Much worse"] : 4
4 : 27.25 : 5
6 : 24.166666666666668 : 5
Reliability IN String["Much worse","better"] : 4
4 : 21.0 : 5
7 : 24.428571428571427 : 5
Type IN String["Medium"] : 3
Reliability IN String["Much better","worse"] : 4
6 : 21.333333333333332 : 5
5 : 22.2 : 5
6 : 19.333333333333332 : 4
```

The labels show the decision that is made at each node. The lines that begin with a number show the number of observations that were placed in that bin along with the average mileage of those observations. The output isn’t pretty but it isn’t that difficult to follow since the tree is pretty shallow.

And, as always, I’ve uploaded my code to Github.