Using Logistic Regression in Python for Data Science
You can use logistic regression in Python for data science. Linear regression is well suited for estimating values, but it isn’t the best tool for predicting the class of an observation. In spite of the statistical theory that advises against it, you can actually try to classify a binary class by scoring one class as 1 and the other as 0. The results are disappointing most of the time, so the statistical theory wasn’t wrong!
The fact is that linear regression works on a continuum of numeric estimates. In order to classify correctly, you need a more suitable measure, such as the probability of class ownership. Thanks to the following formula, you can transform a linear regression numeric estimate into a probability that is more apt to describe how a class fits an observation:
probability of a class = exp(r) / (1+exp(r))
r is the regression result (the sum of the variables weighted by the coefficients) and exp is the exponential function. exp(r) corresponds to Euler’s number e elevated to the power of r. A linear regression using such a formula (also called a link function) for transforming its results into probabilities is a logistic regression.
Applying logistic regression
Logistic regression is similar to linear regression, with the only difference being the y data, which should contain integer values indicating the class relative to the observation. Using the Iris dataset from the Scikit-learn datasets module, you can use the values 0, 1, and 2 to denote three classes that correspond to three species:
from sklearn.datasets import load_iris iris = load_iris() X, y = iris.data[:-1,:], iris.target[:-1]
To make the example easier to work with, leave a single value out so that later you can use this value to test the efficacy of the logistic regression model on it.
from sklearn.linear_model import LogisticRegression logistic = LogisticRegression() logistic.fit(X,y) print ‘Predicted class %s, real class %s’ % ( logistic.predict(iris.data[-1,:]),iris.target[-1]) print ‘Probabilities for each class from 0 to 2: %s’ % logistic.predict_proba(iris.data[-1,:]) Predicted class , real class 2 Probabilities for each class from 0 to 2: [[ 0.00168787 0.28720074 0.71111138]]
Contrary to linear regression, logistic regression doesn’t just output the resulting class (in this case, the class 2), but it also estimates the probability of the observation’s being part of all three classes. Based on the observation used for prediction, logistic regression estimates a probability of 71 percent of its being from class 2 — a high probability, but not a perfect score, therefore leaving a margin of uncertainty.
Using probabilities lets you guess the most probable class, but you can also order the predictions with respect to being part of that class. This is especially useful for medical purposes: Ranking a prediction in terms of likelihood with respect to others can reveal what patients are at most risk of getting or already having a disease.
Considering when classes are more
The previous problem, logistic regression, automatically handles a multiple class problem (it started with three iris species to guess). Most algorithms provided by Scikit-learn that predict probabilities or a score for class can automatically handle multiclass problems using two different strategies:
One versus rest: The algorithm compares every class with all the remaining classes, building a model for every class. If you have ten classes to guess, you have ten models. This approach relies on the OneVsRestClassifier class from Scikit-learn.
One versus one: The algorithm compares every class against every individual remaining class, building a number of models equivalent to n * (n-1) / 2, where n is the number of classes. If you have ten classes, you have 45 models. This approach relies on the OneVsOneClassifier class from Scikit-learn.
In the case of logistic regression, the default multiclass strategy is the one versus rest. This example shows how to use both the strategies with the handwritten digit dataset, containing a class for numbers from 0 to 9. The following code loads the data and places it into variables.
from sklearn.datasets import load_digits digits = load_digits() X, y = digits.data[:1700,:], digits.target[:1700] tX, ty = digits.data[1700:,:], digits.target[1700:]
The observations are actually a grid of pixel values. The grid’s dimensions are 8 pixels by 8 pixels. To make the data easier to learn by machine-learning algorithms, the code aligns them into a list of 64 elements. The example reserves a part of the available examples for a test.
from sklearn.multiclass import OneVsRestClassifier from sklearn.multiclass import OneVsOneClassifier OVR = OneVsRestClassifier(LogisticRegression()).fit(X,y) OVO = OneVsOneClassifier(LogisticRegression()).fit(X,y) print ‘One vs rest accuracy: %.3f’ % OVR.score(tX,ty) print ‘One vs one accuracy: %.3f’ % OVO.score(tX,ty) One vs rest accuracy: 0.938 One vs one accuracy: 0.969
The two multiclass classes OneVsRestClassifier and OneVsOneClassifier operate by incorporating the estimator (in this case, LogisticRegression). After incorporation, they usually work just like any other learning algorithm in Scikit-learn. Interestingly, the one-versus-one strategy obtained the best accuracy thanks to its high number of models in competition.
When working with Anaconda and Python version 3.4, you may receive a deprecation warning when working with this example. You’re safe to ignore the deprecation warning — the example should work as normal. All the deprecation warning tells you is that one of the features used in the example is due for an update or will become unavailable in a future version of Python.