Machine Learning Mastery Series: Part 4
Welcome back to the Machine Learning Mastery Series! In this fourth part, we’ll dive into Logistic Regression, a widely used algorithm for classification tasks. While Linear Regression predicts continuous outcomes, Logistic Regression is designed for binary and multiclass classification.
Understanding Logistic Regression
Logistic Regression is a supervised learning algorithm that models the probability of a binary or multiclass target variable. Unlike Linear Regression, where the output is a continuous value, Logistic Regression outputs the probability of the input data belonging to a specific class.
Sigmoid Function
Logistic Regression uses the sigmoid (logistic) function to transform the output of a linear equation into a probability between 0 and 1. The sigmoid function is defined as:
P(y=1) = 1 / (1 + e^(z))
Where:
P(y=1)
is the probability of the positive class.e
is the base of the natural logarithm.z
is the linear combination of features and coefficients.
Binary Classification
In binary classification, there are two possible classes (0 and 1). The model predicts the probability of an input belonging to the positive class (1). If the probability is greater than a threshold (usually 0.5), the data point is classified as the positive class; otherwise, it’s classified as the negative class (0).
MultiClass Classification
For multiclass classification, Logistic Regression can be extended to predict multiple classes using techniques like onevsrest (OvR) or softmax regression.
Training a Logistic Regression Model
To train a Logistic Regression model, follow these steps:

Data Collection: Gather a labeled dataset with features and target labels (0 or 1 for binary classification, or multiple classes for multiclass classification).

Data Preprocessing: Clean, preprocess, and split the data into training and testing sets.

Model Selection: Choose Logistic Regression as the algorithm for classification.

Training: Fit the model to the training data by estimating the coefficients that maximize the likelihood of the observed data.

Evaluation: Assess the model’s performance on the testing data using evaluation metrics like accuracy, precision, recall, F1score, and ROC AUC.

Prediction: Use the trained model to make predictions on new, unseen data.
Example Use Cases
Logistic Regression is versatile and finds applications in various domains:
 Medical Diagnosis: Predicting disease presence or absence based on patient data.
 Email Spam Detection: Classifying emails as spam or not.
 Credit Risk Assessment: Determining the risk of loan default.
 Sentiment Analysis: Analyzing sentiment in text data (positive, negative, neutral).
 Image Classification: Identifying objects or categories in images.
In the next part of the series, we cover Machine Learning Mastery Series: Part 5 – Decision Trees and Random Forest