In yesterday’s post, we asked the basic question of what is machine learning. I hoped to illustrate the similarities and differences between artificial intelligence and machine learning. Lately, on this site, we have been spending a bit of time using Python and I wanted to take a moment today to look at a great library for machine learning in Python.
Scikit-learn is the go-to library for machine learning with an amazing ecosystem of plugins. It is open-source and supports supervised and unsupervised learning. It also provides various tools for model fitting, data preprocessing, model selection, model evaluation, and many other utilities. After you python3 -m venv EnvironmentName and source EnvironmentName/bin/activate, you can install it by running pip install scikit-learn. At that point, you can reference it in your code as sklearn.
The way that scikit-learn works is that you start with some data, you give it to a model, the model learns from it, and then you will be able to make predictions. The common notation is splitting up the data into a part called X (everything you are using to make a prediction) and another part called Y (the prediction you are interested in making). The X could be information about a house (square feet, number of bathrooms, etc) where Y is the house price, or X could be a patient’s health statistics where Y is whether or not they develop diabetes. The model then uses X to try to predict Y.
In the above code, we load the 20,640 records and 9 columns into the data variable and then we set the things that we are using to make a prediction to X and the prediction that we are interested in making to y. So, what are the feature (column) names for the data? If you print(data.feature_names), it will print them.
sklearn.model_selection
Once you have data, you can start working on creating a model. The model itself is nothing more than a Python object but the goal after you create it is to train it. You will want to split your data into a training set and a test set. Using <a href="https://scikit-learn.org/stable/modules/generated/sklearn.model_selection.train_test_split.html#sklearn.model_selection.train_test_split">train_test_split</a> in sklearn.model_selection, you can split it into 70% of the data for training the model and 30% of the data for testing the model (or whatever split you want).
In the above example, we are taking any X values except num_preg (the number of pregnancies) that have the value 0 and setting it to the mean. That makes it so that missing values don’t scew things when you go to train the model.
The neat thing about .fit() is that if you want to swap out the KNeighborsRegressor model with a new one, .fit() still works just the same. Let’s look at what it would look like using a linear regression model.
In the above code, we are predicting the value for y and then comparing it against the actual value of y. Using just the training data, it is predicting the values with a 75.23% level of accuracy.
So, what is next?
In a future post, I want to step through the whole process of picking a statement to test, adjusting the data, building and training a model, testing, adjusting the model, and making predictions. Let’s save that for another day, though.