Bell-shaped distribution, often referred to as Gaussian. To test the assumption of normality, a variety of methods are available, but the simplest is to inspect the data visually using a histogram or a Q-Q scatterplot. Real-world data are almost never perfectly normal, so this assumption can be considered reasonably met if the shape looks approximately symmetric and bell-shaped. The data in the example figure below is approximately normally distributed.

Normal Distribution

https://machinelearningmastery.com/statistical-data-distributions/

A Gentle Introduction to Statistical Data Distributions

A distribution with constant probability.

Uniform Distribution

When the mean of the distribution is known, and it is also known that the values are all positive, we can assume exponential distribution. 

Exponential Distribution

Data Distributions: A Statistical Handbook (Entropy Minimax Sourcebook)
(https://www.amazon.com/Data-Distributions-Statistical-Handbook-Sourcebook/dp/0938876171)

Data Distributions: A Statistical Handbook (Entropy Minimax Sourcebook)

Distributions are often described by it's density functions.
There are two types of density functions:
1) Probability Density Function (PDF - Discrete)/Probability mass function(PMF-Continuous) 
2) Cumulattive Density Function (CDF)  

The graph below can shows the relationship between this two functions. For PDF, it will give you the probability one event happens. CDF gives you the probability an event equal or less than a value.


Density Function

A bimodal distribution is a specific shape of data distribution characterized by two distinct peaks, representing two scores or categories that occur most frequently. While less common than unimodal distributions in psychological research, they can emerge when different subsets of a sample respond divergently.

Bimodal Distribution

Data can be distributed in several ways. Some of these include:
- Normal (Gaussian) distributions
- Uniform distributions
- Exponential distributions

University of Michigan - Ann Arbor

A dataset is collections of data, which can be in the form of a table, network, text, image, video, sound, ...

Dataset

- Number of data points (instances)
- Attributes (variables)
- Types of attributes (variables)
- Number of attributes (variables)
- Statistical measures

Characteristics of a dataset

Here are a list of free resources to find datasets. Most will be available in CSV format for easy integration into R or Python. 

Sample Datasets

It important to examine your dataset before starting to work with it for a machine learning task to:
- Understand how much missing data there is in the dataset.
- Get an idea of whether the features need further cleaning.
- Understand what techniques to use for analyzing the data.

The first step to analyze a dataset:

- 1) Equilibrium
- 2) Cyclic (periodic)
- 3) Random 
- 4) Complex

Wolfram's four classes of empirical data

Data Distributions

Supervised learning is a type of machine learning problem where the objective is to predict a designated unknown label based on known inputs. This approach requires a training dataset consisting of examples for which the ground truth labels are already known, allowing the model to learn the mapping from inputs to classifications.

Supervised Learning

A machine learning model is the computational machinery designed to ingest data of a specific type, apply mathematical transformations, and output predictions. In modern applications, the focus is on statistical models whose internal transformations can be estimated and refined using a dataset.

Machine Learning Model

Before data can be fed into any machine learning model, its overall quality must be evaluated and improved. Real-world datasets are often plagued by flaws such as extreme outliers, faulty sensor measurements, and human recording errors. Addressing these discrepancies is an essential part of the data preparation process to ensure the model learns from reliable information.

Data Quality

In real-world machine learning scenarios, datasets are often not provided in a single, simple CSV file. Instead, they may be stored in a relational database, where information is distributed across multiple tables. For example, an e-commerce application might store customer addresses in one table and their purchase data in another, requiring practitioners to extract and merge these separate files before modeling.

Relational Database

Beyond standard categorical and numeric variables, deep learning practitioners frequently work with myriad unstructured data formats. These complex data types include text strings for natural language processing, images and point clouds for computer vision, and audio data. Handling these diverse formats requires specialized approaches and advanced tools compared to traditional tabular data.

Deep Learning Data Types

A data processing bottleneck occurs when the time taken to load, transform, and prepare complex data—such as high-resolution images or large text corpora—exceeds the time required for actual model training. To prevent data preprocessing from severely slowing down the overall machine learning pipeline, practitioners must implement efficient algorithms and utilize advanced tools.

Data Processing Bottleneck

The success of a machine learning model heavily depends on having the right data, as encapsulated by the principle 'garbage in, garbage out.' If a dataset contains mistakes, features that do not predict the target, or reflects societal prejudices by under-representing certain groups, the learning process will fail or produce biased, harmful results.

Machine Learning Dataset Quality

A machine learning example, interchangeably referred to as a data point, sample, or instance, represents an individual record within a dataset. To be useful for learning algorithms, each example is characterized by a specific numerical representation, forming the basis for a model's predictions.

Machine Learning Example

Comma-separated values (CSV) files are a ubiquitous format for storing tabular, spreadsheet-like data. In a CSV file, each line represents a distinct data record, and the individual fields within that record are separated by commas.

CSV File

A data batch, commonly referred to as a minibatch, is a discrete, smaller subset of a larger dataset that is processed simultaneously during a single iteration of machine learning model training or evaluation. Training typically requires multiple passes over a dataset, grabbing one minibatch at a time. Each minibatch consists of a tuple of features and corresponding labels. Processing data in minibatches rather than the entire dataset balances computational efficiency with hardware memory constraints.

Data Batch

When loading datasets for machine learning, the order in which data is read depends on the current mode. For training, data is typically read in a random order (shuffled) to properly update the model. For validation, data is read in a pre-defined, sequential order, which is important for debugging purposes and consistent evaluation.

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