What is train and test data in python?
Training and test data are common for supervised learning algorithms. Given a dataset, its split into training set and test set. In Machine Learning, this applies to supervised learning algorithms. Related course:
Complete Machine Learning Course with Python In the real world we have all kinds of data like financial data or customer data. An
algorithm should make new predictions based on new data. You can simulate this by splitting the dataset in training and test data. Code exampleThe module sklearn comes with some datasets. One of these
dataset is the iris dataset. Then the data is split randomly using the method train_test_split.
Download examples A goal of supervised learning is to build a model that performs well on new data. If you have new data, it’s a good idea to see how your model performs on it. The problem is that you may not have new data, but you can simulate this experience with a procedure like train test split. What Is Train Test Split?Train test split is a model validation process that allows you to simulate how your model would perform with new data. This tutorial includes:
If you would like to follow along, the code and images used in this tutorial is available on GitHub. With that, let’s get started. What Is the Train Test Split Procedure?Train test split is a model validation procedure that allows you to simulate how a model would perform on new/unseen data. Here is how the procedure works: Train test split procedure. | Image: Michael Galarnyk1. Arrange the DataMake sure your data is arranged into a format acceptable for train test split. In scikit-learn, this consists of separating your full data set into “Features” and “Target.” 2. Split the DataSplit the data set into two pieces — a training set and a testing set. This consists of random sampling without replacement about 75 percent of the rows (you can vary this) and putting them into your training set. The remaining 25 percent is put into your test set. Note that the colors in “Features” and “Target” indicate where their data will go (“X_train,” “X_test,” “y_train,” “y_test”) for a particular train test split. 3. Train the ModelTrain the model on the training set. This is “X_train” and “y_train” in the image. 4. Test the ModelTest the model on the testing set (“X_test” and “y_test” in the image) and evaluate the performance. More on Data ScienceUnderstanding Boxpolots Consequences of Not Using Train Test SplitYou could try not using train test split and instead train and test the model on the same data. However, I don’t recommend this approach as it doesn’t simulate how a model would perform on new data. It also tends to reward overly complex models that overfit on the data set. The steps below go over how this inadvisable process works. Train and test procedure without splitting the data. | Image: Michael Galarnyk1. Arrange the DataMake sure your data is arranged into a format acceptable for train test split. In scikit-learn, this consists of separating your full data set into “Features” and “Target.” 2. Train the ModelTrain the model on “Features” and “Target.” 3. Test the ModelTest the model on “Features” and “Target” and evaluate the performance. I want to emphasize again that training on an entire data set and then testing on that same data set can lead to overfitting. Overfitting is defined in the image below. The green squiggly line best follows the training data. The problem is that it is likely overfitting on the training data, meaning it is likely to perform worse on new data. Example of overfitted training data. | Image: WikipediaUsing Train Test Split In Python“Change hyperparameters” in this image is also known as hyperparameter tuning. | Image: Michael Galarnyk.This section is about the practical application of train test split as a way to predict home prices. It spans everything from importing a data set to performing a train test split to hyperparameter tuning a decision tree regressor to predicting home prices and more. Python has a lot of libraries that help you accomplish your data science goals including scikit-learn, pandas, and NumPy, which the code below imports.
2. Load the Data SetKaggle hosts a data set which contains the price at which houses were sold for King County, which includes Seattle between May 2014 and May 2015. You can download the data set fromKaggle or load it from my GitHub. The code below loads the data set. Table of home prices. | Image: Michael Galarnyk3. Arrange Data into Features and TargetScikit-learn’s Image: Michael Galarnyk4. Split Data Into Training and Testing SetsThe colors in the image indicate which variable (X_train, X_test, y_train, y_test) from the original dataframe (df) will go to for a particular train test split. If you are curious how the image was made above, I recommend you download and run the ArrangeDataKingCountySplit notebook as pandas styling functionality doesn’t always render on GitHub. | Image: Michael Galarnyk.In
the code below,
The image below shows the number of rows and columns the variables contain using the shape attribute before and after the Shape before and after More on Pandas DataFrameFrom Clipboard to DataFrame With Pandas: A Quick Guide 5. What Is ‘random_state’ in Train Test Split?Image: Michael GalarnykThe image above shows that if you select a different value for There are a number of reasons why people use Train Test Split: Creating and Training a Model in Scikit-LearnHere’s what you need to know: 4 Steps for Train Test Split Creation and Training in Scikit-Learn
1. Import the Model You Want to UseIn scikit-learn, all machine learning models are implemented as Python classes.
2. Make An Instance of the ModelIn the code below, I set the hyperparameter
3. Train the Model on the DataTrain the model on the data, storing the information learned from the data.
4. Predict Labels of Unseen Test Data
For the multiple predictions above, notice how many times some of the predictions are repeated. If you are wondering why, I encourage you to check out the code below, which will start by looking at a single observation/house and then proceed to examine how the model makes its prediction. One house’s features visualized as a Pandas DataFrame. | Image: Michael Galarnyk
The code below shows how to make a prediction for that single observation.
The image below shows how the trained model makes a prediction for the one observation. Image: Michael Galarnyk.If you are curious how these sorts of diagrams are made, consider checking out my tutorial Visualizing Decision Trees using Graphviz and Matplotlib. Measuring Train Test Split Model PerformanceR² (coefficient of determination) formula. | Image: Michael GalarnykWhile there are other ways of measuring model performance such as root-mean-square error, and mean absolute error, we are going to keep this simple and use R² — known as the coefficient of determination — as our metric. The best possible score is 1.0. A constant model that would always predict the mean value of price would get a R² score of 0.0. However, it is possible to get a negative R² on the test set. The code below uses the trained model’s score method to return the R² of the model that we evaluated on the test set.
You might be wondering if our R² above is good for our model. In general, the higher the R², the better the model fits the data. Determining whether a model is performing well can also depend on your field of study. Something harder to predict will generally have a lower R². My argument is that for housing data, we should have a higher R² based solely on our data. Domain experts generally agree that one
of the most important factors in housing prices is location. After all, if you are looking for a home, you’ll most likely care where it’s located. As you can see in the trained model below, the decision tree only incorporates Even
if the model was performing very well, it is unlikely that it would get buy-in from stakeholders or coworkers since there is more to a home than Note that the original data set has location information like “lat” and “long.” The image below visualizes the price percentile of all the houses in the data set based on “lat” and “long,” neither were included in the data the model trained on. As you can see, there is a relationship between home price and location. You can incorporate location information like “lat” and “long” as a way to improve the model. It’s likely places like Zillow found a way to incorporate that into their models. Housing price percentile for King County. | Image: Michael GalarnykHow to Tune the ‘max_depth’ of a TreeThe R² for the model trained earlier in the tutorial was about This involves selecting the optimal values of tuning parameters for a machine learning problem, which are often called hyperparameters. But first, we need to briefly go over the difference between parameters and hyperparameters. Parameters vs HyperparametersA machine learning algorithm estimates model parameters for a given data set and updates these values as it continues to learn. You can think of a model parameter as a learned value from applying the fitting process. For example, in logistic regression you have model coefficients. In a neural network, you can think of neural network weights as a parameter. Hyperparameters or tuning parameters are metaparameters that influence the fitting process itself. For logistic regression, there are many hyperparameters like regularization strength C. For a neural network, there are many hyperparameters like the number of hidden layers. If all of this sounds confusing, Jason Brownlee, founder of Machine Learning Mastery, offers a good rule of thumb in his guide on parameters and hyperparameters which is: “If you have to specify a model parameter manually, then it is probably a model hyperparameter.” Hyperparameter tuningThere are a lot of different ways to hyperparameter tune a decision
tree for regression. One way is to tune the The code below outputs the accuracy for decision trees with different values for
The graph below shows that the best model R² is when the hyperparameter
Image: Michael Galarnyk.Note
that the model above could have still been overfitted on the test set since the code changed More on HyperparametersRandom Forest Algorithm: A Complete Guide Understanding the Bias-Variance TradeoffIn order to understand why Naturally, the training R² is always better than the test R² for every point on this graph because the models are making predictions on data they have seen before. To the left
side of the “Best Model” on the graph (anything less than To the right side of the “Best Model” on the graph (anything more than
The “Best Model” is formed by minimizing bias error — or bad assumptions in the model — and variance error — or oversensitivity to small fluctuations/noise in the training set. Train Test Split Advantages and Disadvantages“Cross-validation: evaluating estimator performance” image from the scikit-learn documentation. | Image: scikit-learnA goal of supervised learning is to build a model that performs well on new data, which train test split helps you simulate. With any model validation procedure it’s important to keep in mind the advantages and disadvantages. The advantages of train test split include:
Its disadvantages include:
What is the difference between train and test data?In machine learning, datasets are split into two subsets. The first subset is known as the training data - it's a portion of our actual dataset that is fed into the machine learning model to discover and learn patterns. In this way, it trains our model. The other subset is known as the testing data.
What is train and test data explain There purposes?Typically, when you separate a data set into a training set and testing set, most of the data is used for training, and a smaller portion of the data is used for testing. SQL Server Analysis Services randomly samples the data to help ensure that the testing and training sets are similar.
What is train_test_split Python?The train_test_split function of the sklearn. model_selection package in Python splits arrays or matrices into random subsets for train and test data, respectively.
What is training data and testing data in deep learning?The main difference between training data and testing data is that training data is the subset of original data that is used to train the machine learning model, whereas testing data is used to check the accuracy of the model.
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