# Tree based approaches

## Decision Tree

A decision tree is a flowchart-like structure in which each internal node represents a "test" on an attribute (e.g., whether a coin flip comes up heads or tails), each branch represents the outcome of the test, and each leaf node represents a class label (decision taken after computing all attributes). It is immensly popular primarily due to its ease of explanation which is often a critical requirement in business.

Decision Trees follow Sum of Product (SOP) representation. The Sum of product (SOP) is also known as Disjunctive Normal Form. For a class, every branch from the root of the tree to a leaf node having the same class is conjunction (product) of values, different branches ending in that class form a disjunction (sum).

The primary challenge in the decision tree implementation is to identify which attributes do we need to consider as the root node and each level. Handling this is to know as the attribute selection. We have different attributes selection measures to identify the attribute which can be considered as the root note at each level.

The decision of making strategic splits heavily affects a tree’s accuracy. The decision criteria are different for classification and regression trees.

Decision trees use multiple algorithms to decide to split a node into two or more sub-nodes. The creation of sub-nodes increases the homogeneity of resultant sub-nodes. In other words, we can say that the purity of the node increases with respect to the target variable. The decision tree splits the nodes on all available variables and then selects the split which results in most homogeneous sub-nodes.

The algorithm selection is also based on the type of target variables. Let us look at some algorithms used in Decision Trees:

ID3 → (extension of D3)

C4.5 → (successor of ID3)

CART → (Classification And Regression Tree)

CHAID → (Chi-square automatic interaction detection Performs multi-level splits when computing classification trees)

MARS → (multivariate adaptive regression splines)

The ID3 algorithm builds decision trees using a top-down greedy search approach through the space of possible branches with no backtracking. A greedy algorithm, as the name suggests, always makes the choice that seems to be the best at that moment.

### Steps in the ID3 algorithm:

It begins with the original set as the root node.

On each iteration of the algorithm, it iterates through the unused attributes of the set and calculates a measure of Homogeneity:

**Gini Index:**Gini Index uses the probability of finding a data point with one label as an indicator for homogeneity — if the dataset is completely homogeneous, then the probability of finding a datapoint with one of the labels is 1 and the probability of finding a data point with the other label is zero**Information Gain / Entropy-based:**The idea is to use the notion of entropy which is a central concept in information theory. Entropy quantifies the degree of disorder in the data. Entropy is always a positive number between zero and 1. Another interpretation of entropy is in terms of information content. A completely homogeneous dataset has no information content in it (there is nothing non-trivial to be learnt from the dataset) whereas a dataset with a lot of disorder has a lot of latent information waiting to be learnt.For Regression models the split can happen by checking metrics like $R^2$

It then selects the attribute which has the smallest Entropy or Largest Information gain

The set is then split by the selected attribute to produce a subset of the data

The algorithm continues to recur on each subset, considering only attributes never selected before

Decision trees have a strong tendency to overfit the data. So practical uses of the decision tree must necessarily incorporate some ’regularization’ measures to ensure the decision tree built does not become more complex than is necessary and starts to overfit. There are broadly two ways of regularization on decision trees:

**Truncation:**Truncate the decision tree during the training (growing) process preventing it from degenerating into one with one leaf for every data point in the training dataset. Below criterion are used to decide if the decision tree needs to be grown further:Minimum Size of the Partition for a Split: Stop partitioning further when the current partition is small enough.

Minimum Change in Homogeneity Measure: Do not partition further when even the best split causes an insignificant change in the purity measure (difference between the current purity and the purity of the partitions created by the split).

Limit on Tree Depth: If the current node is farther away from the root than a threshold, then stop partitioning further.

Minimum Size of the Partition at a Leaf: If any of partitions from a split has fewer than this threshold minimum, then do not consider the split. Notice the subtle difference between this condition and the minimum size required for a split.

Maxmimum number of leaves in the Tree: If the current number of the bottom-most nodes in the tree exceeds this limit then stop partitioning.

**Pruning:**Let the tree grow to any complexity. However add a post-processing step in which we prune the tree in a bottom-up fashion starting from the leaves. It is more common to use pruning strategies to avoid overfitting in practical implementations. One popular approach to pruning is to use a validation set. This method called reduced-error pruning, considers every one of the test (non-leaf ) nodes for pruning. Pruning a node means removing the entire subtree below the node, making it a leaf, and assigning the majority class (or the average of the values in case it is regression) among the training data points that pass through that node. A node in the tree is pruned only if the decision tree obtained after the pruning has an accuracy that is no worse on the validation dataset than the tree prior to pruning. This ensures that parts of the tree that were added due to accidental irregularities in the data are removed, as these irregularities are not likely to repeat.

### Hyperparameters

Though there are various ways to truncate or prune trees, the `DecisionTreeClassifier`

function in sklearn provides the following hyperparameters which you can control:

`criterion (Gini/IG or entropy)`

: It defines the function to measure the quality of a split. Sklearn supports “gini” criteria for Gini Index & “entropy” for Information Gain. By default, it takes the value “gini”.`max_features`

: It defines the no. of features to consider when looking for the best split`max_depth`

: denotes maximum depth of the tree. It can take any integer value or None. If None, then nodes are expanded until all leaves are pure or until all leaves contain less than min_samples_split samples. By default, it takes “None” value.`min_samples_split`

: This tells above the minimum no. of samples reqd. to split an internal node`min_samples_leaf`

: The minimum number of samples required to be at a leaf node

## Random Forest

*Ensemble means a group of things viewed as a whole rather than individually. In ensembles, a collection of models is used to make predictions, rather than individual models. Arguably, the most popular in the family of ensemble models is the random forest: an ensemble made by the combination of a large number of decision trees.*

For an ensemble to work, each model of the ensemble should comply with the following conditions:

Each model should be diverse. Diversity ensures that the models serve complementary purposes, which means that the individual models make predictions independent of each other.

Each model should be acceptable. Acceptability implies that each model is at least better than a random model.

Random forests are created using a special ensemble method called **bagging (Bootstrap Aggregation)**. Bootstrapping means creating bootstrap samples from a given data set. A bootstrap sample is created by sampling the given data set uniformly and with replacement. A bootstrap sample typically contains about $30$-$70$% data from the data set. Aggregation implies combining the results of different models present in the ensemble.

### Steps

Create a bootstrap sample from the training set

Now construct a decision tree using the bootstrap sample. While splitting a node of the tree, only consider a random subset of features. Every time a node has to split, a different random subset of features will be considered.

Repeat the steps 1 and 2 for $n$ times, to construct $n$ trees in the forest. Remember each tree is constructed independently, so it is possible to construct each tree in parallel.

While predicting a test case, each tree predicts individually, and the final prediction is given by the majority vote of all the trees

### OOB (Out-of-Bag) Error

The OOB error is calculated by using each observation of the training set as a test observation. Since each tree is built on a bootstrap sample, each observation can used as a test observation by those trees which did not have it in their bootstrap sample. All these trees predict on this observation and you get an error for a single observation. The final OOB error is calculated by calculating the error on each observation and aggregating it.

It turns out that the OOB error is as good as cross validation error.

### Advantages

A random forest is more stable than any single decision tree because the results get averaged out; it is not affected by the instability and bias of an individual tree.

A random forest is immune to the curse of dimensionality since only a subset of features is used to split a node.

You can parallelize the training of a forest since each tree is constructed independently.

You can calculate the OOB (Out-of-Bag) error using the training set which gives a really good estimate of the performance of the forest on unseen data. Hence there is no need to split the data into training and validation; you can use all the data to train the forest.

### Hyperparameters

`n_estimators`

: The number of trees in the forest.`criterion`

: The function to measure the quality of a split. Supported criteria are “gini” for the Gini impurity and “entropy” for the information gain. This parameter is tree-specific.`max_features`

: The number of features to consider when looking for the best split`max_depth`

: The maximum depth of the tree`min_samples_split`

: The minimum number of samples required to split an internal node`min_samples_leaf`

: The minimum number of samples required to be at a leaf node`min_weight_fraction_leaf`

: The minimum weighted fraction of the sum total of weights (of all the input samples) required to be at a leaf node. Samples have equal weight when sample_weight is not provided`max_leaf_nodes`

: Grow trees with max_leaf_nodes in best-first fashion. Best nodes are defined as relative reduction in impurity`min_impurity_split`

: Threshold for early stopping in tree growth. A node will split if its impurity is above the threshold, otherwise it is a leaf

## Ensemble Learning

Ensemble Learning is called the Wisdom of the crowd. Combine multiple weak models/learners into one predictive model to reduce bias, variance and/or improve accuracy.

**Bagging:**Trains N different weak models (usually of same types – homogenous) with N non-overlapping subset of the input dataset in parallel. In the test phase, each model is evaluated. The label with the greatest number of predictions is selected as the prediction. Bagging methods reduces variance of the prediction**Boosting:**Trains N different weak models (usually of same types – homogenous) with the complete dataset in a sequential order. The datapoints wrongly classified with previous weak model is provided more weights to that they can be classified by the next weak leaner properly. In the test phase, each model is evaluated and based on the test error of each weak model, the prediction is weighted for voting. Boosting methods decreases the bias of the prediction.**Stacking:**Trains N different weak models (usually of different types – heterogenous) with one of the two subsets of the dataset in parallel. Once the weak learners are trained, they are used to trained a meta learner to combine their predictions and carry out final prediction using the other subset. In test phase, each model predicts its label, these set of labels are fed to the meta learner which generates the final prediction.

## Boosting

Boosting was first introduced in 1997 by Freund and Schapire in the popular algorithm, AdaBoost. It was originally designed for classification problems. Since its inception, many new boosting algorithms have been developed those tackle regression problems also and have become famous as they are used in the top solutions of many Kaggle competitions.

An ensemble is a collection of models which ideally should predict better than individual models. The key idea of boosting is to create an ensemble which makes high errors only on the less frequent data points. Boosting leverages the fact that we can build a series of models specifically targeted at the data points which have been incorrectly predicted by the other models in the ensemble. If a series of models keep reducing the average error, we will have an ensemble having extremely high accuracy. Boosting is a way of generating a strong model from a weak learning algorithm.

## AdaBoost

AdaBoost starts by assigning equal weight to each datapoint, the idea is to adjust the weights of each observation after every iteration such that the the algorithm is forced to take a harder look at these difficult to classify observations

Post each iteration we will have a weak learner using which we will calculate 2 things:

the updated weights of each $N$ observation for the next iteration

the weight that the weak learner itself will have on the final output, in each of the $t$ iterations we will have a learner $h_1$, $h_2$, $h_3$ .. $h_t$ each of which will be combined to make the final model, the weight of each of these individual learners in the final output is given by $\alpha_t$. The models with low error rate will have higher values of $\alpha_t$ and hence higher weight in the final output.

Before you apply the AdaBoost algorithm, you should specifically remove the Outliers. Since AdaBoost tends to boost up the probabilities of misclassified points and there is a high chance that outliers will be misclassified, it will keep increasing the probability associated with the outliers and make the progress difficult.

## Gradient Boosting

As always let's start with a crude initial function F₀, something like average of all values in case of regression. It will give us some output, however bad.

Calculate the loss function

Next, we should have fit a new model on the residuals given by the Loss function, but there is a subtle twist: we will instead fit on the negative gradient of the loss function (for mathematical proof check the link

This process of fitting the model iteratively on the -ve gradient will continue till we have reached the minima or the limit of the number of weak learners given by T, this is called the additive approach

Recall that, in Adaboost,“shortcomings” are identified by high-weight data points. In Gradient Boosting, “shortcomings” are identified by gradients. This is in short of the intuition as to how Gradient Boosting works. In case of regression and classification the only thing that differs is the loss function that is used.

## XGBoost

XGBoost stands for "Extreme Gradient Boosting", where the term "Gradient Boosting" originates from the paper Greedy Function Approximation: A Gradient Boosting Machine, by Friedman. It initially started as a research project by Tianqi Chen as part of the Distributed (Deep) Machine Learning Community (DMLC) group. It became well known in the ML competition circles after its use in the winning solution of the Higgs Machine Learning Challenge.

XGBoost and GBM both follow the principle of gradient boosted trees, but XGBoost uses a more regularized (by taking the model complexity into account) model formulation to control over-fitting, which gives it better performance, which is why it’s also known as ‘regularized boosting’ technique. In Stochastic Gradient Descent, used by Gradient Boosting, we use less point to take less time to compute the direction we should go towards, in order to make more of them, in the hope we go there quicker. In Newton’s method, used by XGBoost, we take more time to compute the direction we want to go into, in the hope we have to take fewer steps in order to get there.

**Why is XGBoost so good?**

*Parallel Computing:*when you run xgboost, by default, it would use all the cores of your laptop/machine enabling its capacity to do parallel computation*Regularization:*The biggest advantage of xgboost is that it uses regularization and controls the overfitting and simplicity of the model which gives it better performance.*Enabled Cross Validation:*XGBoost is enabled with internal Cross Validation function*Missing Values:*XGBoost is designed to handle missing values internally. The missing values are treated in such a manner that if there exists any trend in missing values, it is captured by the model*Flexibility:*XGBoost is not just limited to regression, classification, and ranking problems, it supports user-defined objective functions as well. Furthermore, it supports user-defined evaluation metrics as well.

## Questions

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