Introduction to Machine Learning Elective Course
- 1. COURSE NAME: INTRODUCTION TO MACHINE LEARNING (ELECTIVE) CREDITS: 3 ACADEMIC YEAR: 2022-23 (EVEN SEMESTER) 1
- 2. MODULE 3 2
- 3. SYLLABUS � A Tour of Machine Learning Classifiers Using Scikit- learn � Choosing a classification algorithm First steps with scikit-learn Training a perception via scikit-learn � Modeling class probabilities via logistic regression Logistic regression intuition and conditional probabilities Learning the weights of the logistic cost function Training a logistic regression model with scikit-learn Tackling over fitting via regularization. � 3
- 4. SYLLABUS � Maximum margin classification with support vector machines Maximum margin intuition Dealing with the nonlinearly separable case using slack variables Alternative implementations in scikit-learn � Solving nonlinear problems using a kernel SVM Using the kernel trick to find separating hyper planes in higher dimensional space � Decision tree learning Maximizing information gain – getting the most bang for the buck Building a decision tree Combining weak to strong learners via random forests � Self Learning Exercise: K-nearest neighbors – a lazy learning algorithm 4
- 5. A TOUR OF MACHINE LEARNING CLASSIFIERS USING SCIKIT-LEARN � There are popular and powerful ML algorithms that are commonly used in academia as well as in industry. � While learning about the differences between several supervised learning algorithms for classification, we will also develop an intuitive appreciation of their individual strengths and weaknesses. � The scikit-learn library, which offers a user- friendly interface for using those algorithms efficiently and productively. 5
- 6. � Robust and popular algorithms for classification, such as ⚫ logistic regression, ⚫ support vector machines, and ⚫ decision trees � Examples and explanations using the scikit-learn machine learning library, which provides a wide variety of machine learning algorithms via a user-friendly Python API � Discussions about the strengths and weaknesses of classifiers with linear and nonlinear decision boundaries 6
- 7. CHOOSING A CLASSIFICATION ALGORITHM � To restate the no free lunch theorem by David H. Wolpert, no single classifier works best across all possible scenarios. � In practice, it is always recommended that you compare the performance of at least a handful of different learning algorithms to select the best model for the particular problem; � these may differ in the number of features or examples, the amount of noise in a dataset, and whether the classes are linearly separable or not. 7
- 8. � The performance of a classifier—computational performance as well as predictive power— depends heavily on the underlying data that is available for learning � The five main steps that are involved in training a supervised machine learning algorithm 1. Selecting features and collecting labeled training examples. 2. Choosing a performance metric. 3. Choosing a classifier and optimization algorithm. 4. Evaluating the performance of the model. 5. Tuning the algorithm. 8
- 9. FIRST STEPS WITH SCIKIT-LEARN – TRAINING A PERCEPTRON � In Module 2, Training Simple Machine Learning Algorithms for Classification, the perceptron rule and Adaline, which is implemented in Python and NumPy. � Now consider the scikit-learn API, which, combines a user-friendly and consistent interface with a highly optimized implementation of several classification algorithms. � The scikit-learn library offers not only a large variety of learning algorithms, but also many convenient functions to preprocess data and to fine-tune and evaluate our models. 9
- 10. � To get started with the scikit-learn library, we will train a perceptron model similar to the one that we implemented in Module 2. � For simplicity, we will use the already familiar Iris dataset . � we will only use two features from the Iris dataset for visualization purposes. � We will assign the petal length and petal width of the 150 flower examples to the feature matrix, X, and the corresponding class labels of the flower species to the vector array, y: 10
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- 19. LOGISTIC REGRESSION IN MACHINE LEARNING � Supervised Learning technique used for predicting the categorical dependent variable using a given set of independent variables. � it gives the probabilistic values which lie between 0 and 1. � Linear Regression is used for solving Regression problems, whereas Logistic regression is used for solving the classification problems. 19
- 20. � Logistic Regression can be used to classify the observations using different types of data and can easily determine the most effective variables used for the classification. The below image is showing the logistic function: 20
- 21. LOGISTIC FUNCTION (SIGMOID FUNCTION): � The sigmoid function is a mathematical function used to map the predicted values to probabilities. � It maps any real value into another value within a range of 0 and 1. � The value of the logistic regression must be between 0 and 1, so it forms a curve like the "S" form. � The S-form curve is called the Sigmoid function or the logistic function. � In logistic regression, we use the concept of the threshold value, which defines the probability of either 0 or 1. 21
- 23. � Type of Logistic Regression: � On the basis of the categories, Logistic Regression can be classified into three types: � Binomial: In binomial Logistic regression, there can be, Pass or Fail, etc. only two possible types of the dependent variables, such as 0 or 1. � Multinomial: In multinomial Logistic regression, there can be 3 or more possible unordered types of the dependent variable, such as "cat", "dogs", or "sheep“ � Ordinal: In ordinal Logistic regression, there can be 3 or more possible ordered types of dependent variables, such as "low", "Medium", or "High". 23
- 24. SUPPORT VECTOR MACHINE ALGORITHM � SVM is Supervised ML algorithms, which is used for Classification as well as Regression problems. � � The goal of the SVM algorithm is to create the best line or decision boundary that can segregate n-dimensional space into classes. This best decision boundary is called a hyperplane. � SVM chooses the extreme points/vectors that help in creating the hyperplane. � These extreme cases are called as support vectors, and hence algorithm is termed as Support Vector Machine. 24
- 25. 25 Consider the below diagram in which there are two different categories that are classified using a decision boundary or hyperplane:
- 26. 26 SVM algorithm can be used for Face detection, image classification, text categorization, etc.
- 27. TYPES OF SVM � SVM can be of two types: � Linear SVM: Linear SVM is used for linearly separable data, which means if a dataset can be classified into two classes by using a single straight line, then such data is termed as linearly separable data, and classifier is used called as Linear SVM classifier. � Non-linear SVM: Non-Linear SVM is used for non- linearly separated data, which means if a dataset cannot be classified by using a straight line, then such data is termed as non-linear data and classifier used is called as Non-linear SVM classifier. 27
- 28. HYPERPLANE AND SUPPORT VECTORS IN THE SVM ALGORITHM: � Hyperplane: There can be multiple lines/decision boundaries to segregate the classes in n-dimensional space, but we need to find out the best decision boundary that helps to classify the data points. This best boundary is known as the hyperplane of SVM. � The dimensions of the hyperplane depend on the features present in the dataset, which means if there are 2 features (as shown in image), then hyperplane will be a straight line. And if there are 3 features, then hyperplane will be a 2-dimension plane. � We always create a hyperplane that has a maximum margin, which means the maximum distance between the data points. 28
- 29. SUPPORT VECTORS: � The data points or vectors that are the closest to the hyperplane and which affect the position of the hyperplane are termed as Support Vector. � Since these vectors support the hyperplane, hence called a Support vector. 29
- 30. HOW DOES SVM WORKS? � Linear SVM: � The working of the SVM algorithm can be understood by using an example. � Suppose we have a dataset that has two tags (green and blue), and the dataset has two features x1 and x2. 30
- 31. 31 We want a classifier that can classify the pair(x1, x2) of coordinates in either green or blue. Consider the below image:
- 32. � So as it is 2-d space so by just using a straight line, we can easily separate these two classes. But there can be multiple lines that can separate these classes. Consider the below image: 32
- 33. � Hence, the SVM algorithm helps to find the best line or decision boundary; this best boundary or region is called as a hyperplane. � SVM algorithm finds the closest point of the lines from both the classes. These points are called support vectors. � The distance between the vectors and the hyperplane is called as margin. � And the goal of SVM is to maximize this margin. � The hyperplane with maximum margin is called the optimal hyperplane. 33
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- 35. � Non-Linear SVM: � If data is linearly arranged, then we can separate it by using a straight line, but for non-linear data, we cannot draw a single straight line. Consider the below image: 35
- 36. � So to separate these data points, we need to add one more dimension. For linear data, we have used two dimensions x and y, so for non-linear data, we will add a third dimension z. � It can be calculated as: z=x2 +y2 � By adding the third dimension, the sample space will become as below image: 36
- 37. � So now, SVM will divide the datasets into classes in the following way. Consider the below image: 37
- 38. � Since we are in 3-d Space, hence it is looking like a plane parallel to the x-axis. If we convert it in 2d space with z=1, then it will become as: � Hence we get a circumference of radius 1 in case of non-linear data. 38
- 39. � Python Implementation of Support Vector Machine � Now we will implement the SVM algorithm using Python. Here we will use the same dataset user_data, which we have used in Logistic regression and KNN classification. � Data Pre-processing step � Till the Data pre-processing step, the code will remain the same. Below is the code: https://www.javatpoint.com/machine-learning-support- vector-machine-algorithm 39
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- 42. MODELING CLASS PROBABILITIES VIA LOGISTIC REGRESSION 42