Machine Learning Models

A machine learning model is defined as a mathematical representation of the output of the training process. Machine learning is the study of different algorithms that can improve automatically through experience & old data and build the model. A machine learning model is similar to computer software designed to recognize patterns or behaviors based on previous experience or data. The learning algorithm discovers patterns within the training data, and it outputs an ML model which captures these patterns and makes predictions on new data.

Let's understand an example of the ML model where we are creating an app to recognize the user's emotions based on facial expressions. So, creating such an app is possible by Machine learning models where we will train a model by feeding images of faces with various emotions labeled on them. Whenever this app is used to determine the user's mood, it reads all fed data then determines any user's mood.

Hence, in simple words, we can say that a machine learning model is a simplified representation of something or a process. In this topic, we will discuss different machine learning models and their techniques and algorithms.

What is Machine Learning Model?

Machine Learning models can be understood as a program that has been trained to find patterns within new data and make predictions. These models are represented as a mathematical function that takes requests in the form of input data, makes predictions on input data, and then provides an output in response. First, these models are trained over a set of data, and then they are provided an algorithm to reason over data, extract the pattern from feed data and learn from those data. Once these models get trained, they can be used to predict the unseen dataset.

There are various types of machine learning models available based on different business goals and data sets.

Classification of Machine Learning Models:

Based on different business goals and data sets, there are three learning models for algorithms. Each machine learning algorithm settles into one of the three models:

• Supervised Learning
• Unsupervised Learning
• Reinforcement Learning

Supervised Learning is further divided into two categories:

• Classification
• Regression

Unsupervised Learning is also divided into below categories:

• Clustering
• Association Rule
• Dimensionality Reduction

1. Supervised Machine Learning Models

Supervised Learning is the simplest machine learning model to understand in which input data is called training data and has a known label or result as an output. So, it works on the principle of input-output pairs. It requires creating a function that can be trained using a training data set, and then it is applied to unknown data and makes some predictive performance. Supervised learning is task-based and tested on labeled data sets.

We can implement a supervised learning model on simple real-life problems. For example, we have a dataset consisting of age and height; then, we can build a supervised learning model to predict the person's height based on their age.

Supervised Learning models are further classified into two categories:

Regression

In regression problems, the output is a continuous variable. Some commonly used Regression models are as follows:

a) Linear Regression

Linear regression is the simplest machine learning model in which we try to predict one output variable using one or more input variables. The representation of linear regression is a linear equation, which combines a set of input values(x) and predicted output(y) for the set of those input values. It is represented in the form of a line:

Y = bx+ c.

The main aim of the linear regression model is to find the best fit line that best fits the data points.

Linear regression is extended to multiple linear regression (find a plane of best fit) and polynomial regression (find the best fit curve).

b) Decision Tree

Decision trees are the popular machine learning models that can be used for both regression and classification problems.

A decision tree uses a tree-like structure of decisions along with their possible consequences and outcomes. In this, each internal node is used to represent a test on an attribute; each branch is used to represent the outcome of the test. The more nodes a decision tree has, the more accurate the result will be.

The advantage of decision trees is that they are intuitive and easy to implement, but they lack accuracy.

Decision trees are widely used in operations research, specifically in decision analysis, strategic planning, and mainly in machine learning.

c) Random Forest

Random Forest is the ensemble learning method, which consists of a large number of decision trees. Each decision tree in a random forest predicts an outcome, and the prediction with the majority of votes is considered as the outcome.

A random forest model can be used for both regression and classification problems.

For the classification task, the outcome of the random forest is taken from the majority of votes. Whereas in the regression task, the outcome is taken from the mean or average of the predictions generated by each tree.

d) Neural Networks

Neural networks are the subset of machine learning and are also known as artificial neural networks. Neural networks are made up of artificial neurons and designed in a way that resembles the human brain structure and working. Each artificial neuron connects with many other neurons in a neural network, and such millions of connected neurons create a sophisticated cognitive structure.

Neural networks consist of a multilayer structure, containing one input layer, one or more hidden layers, and one output layer. As each neuron is connected with another neuron, it transfers data from one layer to the other neuron of the next layers. Finally, data reaches the last layer or output layer of the neural network and generates output.

Neural networks depend on training data to learn and improve their accuracy. However, a perfectly trained & accurate neural network can cluster data quickly and become a powerful machine learning and AI tool. One of the best-known neural networks is Google's search algorithm.

Classification

Classification models are the second type of Supervised Learning techniques, which are used to generate conclusions from observed values in the categorical form. For example, the classification model can identify if the email is spam or not; a buyer will purchase the product or not, etc. Classification algorithms are used to predict two classes and categorize the output into different groups.

In classification, a classifier model is designed that classifies the dataset into different categories, and each category is assigned a label.

There are two types of classifications in machine learning:

• Binary classification: If the problem has only two possible classes, called a binary classifier. For example, cat or dog, Yes or No,
• Multi-class classification: If the problem has more than two possible classes, it is a multi-class classifier.

Some popular classification algorithms are as below:

a) Logistic Regression

Logistic Regression is used to solve the classification problems in machine learning. They are similar to linear regression but used to predict the categorical variables. It can predict the output in either Yes or No, 0 or 1, True or False, etc. However, rather than giving the exact values, it provides the probabilistic values between 0 & 1.

b) Support Vector Machine

Support vector machine or SVM is the popular machine learning algorithm, which is widely used for classification and regression tasks. However, specifically, it is used to solve classification problems. The main aim of SVM is to find the best decision boundaries in an N-dimensional space, which can segregate data points into classes, and the best decision boundary is known as Hyperplane. SVM selects the extreme vector to find the hyperplane, and these vectors are known as support vectors.

c) Naïve Bayes

Naïve Bayes is another popular classification algorithm used in machine learning. It is called so as it is based on Bayes theorem and follows the naïve(independent) assumption between the features which is given as:

Each naïve Bayes classifier assumes that the value of a specific variable is independent of any other variable/feature. For example, if a fruit needs to be classified based on color, shape, and taste. So yellow, oval, and sweet will be recognized as mango. Here each feature is independent of other features.

2. Unsupervised Machine learning models

Unsupervised Machine learning models implement the learning process opposite to supervised learning, which means it enables the model to learn from the unlabeled training dataset. Based on the unlabeled dataset, the model predicts the output. Using unsupervised learning, the model learns hidden patterns from the dataset by itself without any supervision.

Unsupervised learning models are mainly used to perform three tasks, which are as follows:

• Clustering
  Clustering is an unsupervised learning technique that involves clustering or groping the data points into different clusters based on similarities and differences. The objects with the most similarities remain in the same group, and they have no or very few similarities from other groups.
  Clustering algorithms can be widely used in different tasks such as Image segmentation, Statistical data analysis, Market segmentation, etc.
  Some commonly used Clustering algorithms are K-means Clustering, hierarchal Clustering, DBSCAN, etc.
  
• Association Rule Learning
  Association rule learning is an unsupervised learning technique, which finds interesting relations among variables within a large dataset. The main aim of this learning algorithm is to find the dependency of one data item on another data item and map those variables accordingly so that it can generate maximum profit. This algorithm is mainly applied in Market Basket analysis, Web usage mining, continuous production, etc.
  Some popular algorithms of Association rule learning are Apriori Algorithm, Eclat, FP-growth algorithm.
• Dimensionality Reduction
  The number of features/variables present in a dataset is known as the dimensionality of the dataset, and the technique used to reduce the dimensionality is known as the dimensionality reduction technique.
  Although more data provides more accurate results, it can also affect the performance of the model/algorithm, such as overfitting issues. In such cases, dimensionality reduction techniques are used.
  "It is a process of converting the higher dimensions dataset into lesser dimensions dataset ensuring that it provides similar information."
  Different dimensionality reduction methods such as PCA(Principal Component Analysis), Singular Value Decomposition, etc.

Reinforcement Learning

In reinforcement learning, the algorithm learns actions for a given set of states that lead to a goal state. It is a feedback-based learning model that takes feedback signals after each state or action by interacting with the environment. This feedback works as a reward (positive for each good action and negative for each bad action), and the agent's goal is to maximize the positive rewards to improve their performance.

The behavior of the model in reinforcement learning is similar to human learning, as humans learn things by experiences as feedback and interact with the environment.

Below are some popular algorithms that come under reinforcement learning:

• Q-learning: Q-learning is one of the popular model-free algorithms of reinforcement learning, which is based on the Bellman equation.

It aims to learn the policy that can help the AI agent to take the best action for maximizing the reward under a specific circumstance. It incorporates Q values for each state-action pair that indicate the reward to following a given state path, and it tries to maximize the Q-value.

• State-Action-Reward-State-Action (SARSA): SARSA is an On-policy algorithm based on the Markov decision process. It uses the action performed by the current policy to learn the Q-value. The SARSA algorithm stands for State Action Reward State Action, which symbolizes the tuple (s, a, r, s', a').
• Deep Q Network: DQN or Deep Q Neural network is Q-learning within the neural network. It is basically employed in a big state space environment where defining a Q-table would be a complex task. So, in such a case, rather than using Q-table, the neural network uses Q-values for each action based on the state.

Training Machine Learning Models

Once the Machine learning model is built, it is trained in order to get the appropriate results. To train a machine learning model, one needs a huge amount of pre-processed data. Here pre-processed data means data in structured form with reduced null values, etc. If we do not provide pre-processed data, then there are huge chances that our model may perform terribly.

How to choose the best model?

In the above section, we have discussed different machine learning models and algorithms. But one most confusing question that may arise to any beginner that "which model should I choose?". So, the answer is that it depends mainly on the business requirement or project requirement. Apart from this, it also depends on associated attributes, the volume of the available dataset, the number of features, complexity, etc. However, in practice, it is recommended that we always start with the simplest model that can be applied to the particular problem and then gradually enhance the complexity & test the accuracy with the help of parameter tuning and cross-validation.

Difference between Machine learning model and Algorithms

One of the most confusing questions among beginners is that are machine learning models, and algorithms are the same? Because in various cases in machine learning and data science, these two terms are used interchangeably.

The answer to this question is No, and the machine learning model is not the same as an algorithm. In a simple way, an ML algorithm is like a procedure or method that runs on data to discover patterns from it and generate the model. At the same time, a machine learning model is like a computer program that generates output or makes predictions. More specifically, when we train an algorithm with data, it becomes a model.