The purpose of this research is to put together the 7 most commonly used classification algorithms along with the python code: Logistic Regression, Naïve Bayes, Stochastic Gradient Descent, K-Nearest Neighbours, Decision Tree, Random Forest, and Support Vector Machine

# 1 Introduction

## 1.1 Structured Data Classification

Classification can be performed on structured or unstructured data. Classification is a technique where we categorize data into a given number of classes. The main goal of a classification problem is to identify the category/class to which a new data will fall under.

Few of the terminologies encountered in machine learning – classification:

**Classifier:**An algorithm that maps the input data to a specific category.**Classification model:**A classification model tries to draw some conclusion from the input values given for training. It will predict the class labels/categories for the new data.**Feature:**A feature is an individual measurable property of a phenomenon being observed.**Binary Classification:**Classification task with two possible outcomes. Eg: Gender classification (Male / Female)**Multi class classification:**Classification with more than two classes. In multi class classification each sample is assigned to one and only one target label. Eg: An animal can be cat or dog but not both at the same time**Multi label classification:**Classification task where each sample is mapped to a set of target labels (more than one class). Eg: A news article can be about sports, a person, and location at the same time.

The following are the steps involved in building a classification model:

**Initialize**the classifier to be used.**Train the classifier:**All classifiers in scikit-learn uses a fit(X, y) method to fit the model(training) for the given train data X and train label y.**Predict the target:**Given an unlabeled observation X, the predict(X) returns the predicted label y.**Evaluate**the classifier model

## 1.2 Dataset Source and Contents

The dataset contains salaries. The following is a description of our dataset:

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**of Classes:**2 (‘>50K’ and ‘<=50K’)**of attributes (Columns):**7**of instances (Rows):**48,842

This data was extracted from the census bureau database found at:

**http://www.census.gov/ftp/pub/DES/www/welcome.html**

## 1.3 Exploratory Data Analysis

# 2 Classification Algorithms (Python)

## 2.1 Logistic Regression

**Definition: **Logistic regression is a machine learning algorithm for classification. In this algorithm, the probabilities describing the possible outcomes of a single trial are modelled using a logistic function.

**Advantages:** Logistic regression is designed for this purpose (classification), and is most useful for understanding the influence of several independent variables on a single outcome variable.

**Disadvantages:** Works only when the predicted variable is binary, assumes all predictors are independent of each other, and assumes data is free of missing values.

## 2.2 Naïve Bayes

**Definition: **Naive Bayes algorithm based on Bayes’ theorem with the assumption of independence between every pair of features. Naive Bayes classifiers work well in many real-world situations such as document classification and spam filtering.

**Advantages: **This algorithm requires a small amount of training data to estimate the necessary parameters. Naive Bayes classifiers are extremely fast compared to more sophisticated methods.

**Disadvantages: **Naive Bayes is is known to be a bad estimator.

## 2.3 Stochastic Gradient Descent

**Definition: **Stochastic gradient descent is a simple and very efficient approach to fit linear models. It is particularly useful when the number of samples is very large. It supports different loss functions and penalties for classification.

**Advantages:** Efficiency and ease of implementation.

**Disadvantages:** Requires a number of hyper-parameters and it is sensitive to feature scaling.

## 2.4 K-Nearest Neighbours

**Definition: **Neighbours based classification is a type of lazy learning as it does not attempt to construct a general internal model, but simply stores instances of the training data. Classification is computed from a simple majority vote of the k nearest neighbours of each point.

**Advantages: **This algorithm is simple to implement, robust to noisy training data, and effective if training data is large.

**Disadvantages: **Need to determine the value of K and the computation cost is high as it needs to computer the distance of each instance to all the training samples.

## 2.5 Decision Tree

**Definition:** Given a data of attributes together with its classes, a decision tree produces a sequence of rules that can be used to classify the data.

**Advantages:** Decision Tree is simple to understand and visualise, requires little data preparation, and can handle both numerical and categorical data.

**Disadvantages: **Decision tree can create complex trees that do not generalise well, and decision trees can be unstable because small variations in the data might result in a completely different tree being generated.

## 2.6 Random Forest

**Definition: **Random forest classifier is a meta-estimator that fits a number of decision trees on various sub-samples of datasets and uses average to improve the predictive accuracy of the model and controls over-fitting. The sub-sample size is always the same as the original input sample size but the samples are drawn with replacement.

**Advantages: **Reduction in over-fitting and random forest classifier is more accurate than decision trees in most cases.

**Disadvantages: **Slow real time prediction, difficult to implement, and complex algorithm.

## 2.7 Support Vector Machine

**Definition: **Support vector machine is a representation of the training data as points in space separated into categories by a clear gap that is as wide as possible. New examples are then mapped into that same space and predicted to belong to a category based on which side of the gap they fall.

**Advantages: **Effective in high dimensional spaces and uses a subset of training points in the decision function so it is also memory efficient.

**Disadvantages: **The algorithm does not directly provide probability estimates, these are calculated using an expensive five-fold cross-validation.

# 3 Conclusion

## 3.1 Comparison Matrix

**Accuracy: (True Positive + True Negative) / Total Population**- Accuracy is a ratio of correctly predicted observation to the total observations. Accuracy is the most intuitive performance measure.
- True Positive: The number of correct predictions that the occurrence is positive
- True Negative: The number of correct predictions that the occurrence is negative

**F1-Score: (2 x Precision x Recall) / (Precision + Recall)**- F1-Score is the weighted average of Precision and Recall. Therefore, this score takes both false positives and false negatives into account. F1-Score is usually more useful than accuracy, especially if you have an uneven class distribution.
- Precision: When a positive value is predicted, how often is the prediction correct?
- Recall: When the actual value is positive, how often is the prediction correct?

Classification Algorithms | Accuracy | F1-Score |

Logistic Regression | 84.60% | 0.6337 |

Naïve Bayes | 80.11% | 0.6005 |

Stochastic Gradient Descent | 82.20% | 0.5780 |

K-Nearest Neighbours | 83.56% | 0.5924 |

Decision Tree | 84.23% | 0.6308 |

Random Forest | 84.33% | 0.6275 |

Support Vector Machine | 84.09% | 0.6145 |

Code location: https://github.com/f2005636/Classification

## 3.2 Algorithm Selection

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