learn-co-students / dsc-3-32-05-gridsearchcv-al

GridSearchCV

Introduction

In this lesson, we'll explore the concept of parameter tuning to maximize our model performance using a Combinatorial Grid Search!

Objectives

You will be able to:

• Understand and explain parameter tuning and why it is necessary
• Design and create a parameter grid for use with sklearn's GridSearchCV module
• Use GridSearchCV to increase model performance through parameter tuning

Parameter Tuning

By now, you've seen that the process of building and training a supervised learning model is an iterative one. Your first model rarely performs the best! There are multiple ways that we potentially improve model performance. Thus far, most of the techniques we've used have been focused on our data. We can get better data, or more data, or both. We can engineer certain features, or clean up or data by removing things that hurt model performance, like multicollinearity.

The other major way to potentially improve model performance is to find good parameters to set when creating the model. This can have a huge impact on the model's fitness! For example, if we allow a Decision Tree to have too many leaves, the model will almost certainly overfit the data. Too few, and the model will underfit. However, each modeling problem is unique--the same parameters could cause either of those situations, depending on the data, the task at hand, and the complexity of the model needed to best fit the data.

In this lesson, we'll learn how we can use a Combinatorial Grid Search to find the best combination of parameters for a given model.

Grid Searching

When we set parameters in a model, the parameters are not independent of one another--the value set for another parameter can have significant effects on other parameters, thereby affecting overall model performance. Consider the following grid.

Parameter	1	2	3	4
criterion	"gini"	"entropy"
max_depth	1	2	5	10
min_samples_split	1	5	10	20

All the parameters above work together to create the framework of the Decision Tree that will be trained. In a given problem, it may be the case that increasing the value of the parameter for min_samples_split generally improves model performance up to a certain point, by reducing overfitting. However, if the value for max_depth is too low or too high, this may doom the model to overfitting or underfitting, by having a tree with too many arbitrary levels and splits that overfit on noise, or limiting the model to nothing more that a "stump" by only allowing it to grow to one or two levels.

So how do we know which combination of parameters is best? The only way we can really know is to try every single combination! For this reason, grid searching is sometimes referred to an as Exhaustive Search.

Using GridSearchCV

the sklearn library provides an easy way tune model parameters through exhaustive search by using its gridseachcv package, which can be found inside the model_selection module. GridsearchCV combined K-Fold Cross Validation with a grid search of parameters. In order to do this, we must first create a Parameter Grid that tells sklearn which parameters to tune, and which values to try for each of those parameters. GridsearchCV Documentation

clf = DecisionTreeClassifier()

param_grid = {
    "criterion": ["gini", "entropy"],
    "max_depth": [1, 2, 5, 10],
    "min_samples_split": [1, 5, 10, 20]
}

gs_tree = GridSearchCV(clf, param_grid, cv=3)
gs_tree.fit(train_data, train_labels)

gs_tree.best_params_

The following code snippet demonstrates how to use GridSearchCV to perform a parameter grid search using the example parameter grid we created above. We just pass in the classifier that we want to use, as well as parameter grid. Our parameter grid should be a dictionary, where the keys are the parameter names, and the values are the different parameter values we want to use in our gridsearch for each given key. We can also use K-fold cross validation during this process, as denoted by the cv= parameter. In this case, we choose to use 3-fold cross validation for each model created inside our grid search.

This code will run every combination the parameters above. It will start by creating by creating DecisionTreeClassifier(criterion='gini', max_depth=1, min_samples_split=1), training on the data with 3-fold cross validation, and recording the average score. Then, it will change 1 parameter, and repeat the process (e.g. DecisionTreeClassifier(criterion='gini', max_depth=1, min_samples_split=5), and so on), keeping track of the overall performance of each model. Once it has tried every combination, the GridSearchCV object we created will automatically default the model that had the best score. We can even access the best combination of parameters by checking the best_params_ attribute!

Drawbacks of GridSearchCV

GridSearchCV is a great tool for finding the best combination of parameters. However, GridSearchCV is only as good as the parameters we put in our parameter grid--be sure to be thoughtful during this step, and think about which parameters you should test!

The main drawback of of an exhaustive search such as GridsearchCV is that there's no way of telling what's best until we we've exhausted all possibilities! This means training many versions of the same machine learning model, which can be very time consuming and computationally expensive. Consider the example code above--we have 3 parameters, with 2, 4, and 4 parameters to try, respectively. We also set the model to use cross-validation with a value of 3, meaning that each model will be built 3 times, and their performances averaged together. If we do some simple math, we can see that the simple grid search we see above actually results in 2 * 4 * 4 * 3 = 96 different models trained! For projects that involve complex models and/or very large datasets, the time needed to run a gridsearch can often be prohibitive. For this reason, be very thoughtful about the parameters you set--sometimes, the extra runtime isn't worth it--especially when there's no guarantee that the model performance will improve!

Summary

In this lesson, you learned about the concept of grid search, how to perform grid search and the drawbacks associated with the method!