# What is Bayesian probability?

by Stephen M. Walker II, Co-Founder / CEO

## What is Bayesian probability?

Bayesian probability is an interpretation of the concept of probability, where probability is interpreted as a reasonable expectation representing a state of knowledge or as quantifiable uncertainty about a proposition whose truth or falsity is unknown. This interpretation is named after Thomas Bayes, who proved a special case of what is now called Bayes' theorem.

In the Bayesian view, a probability is assigned to a hypothesis, whereas under frequentist inference, a hypothesis is typically tested without being assigned a probability. Bayesian probability belongs to the category of evidential probabilities. To evaluate the probability of a hypothesis, the Bayesian probabilist specifies a prior probability, which is then updated to a posterior probability in light of new evidence.

Bayes' theorem, a fundamental part of Bayesian probability, describes the probability of an event based on prior knowledge of conditions that might be related to the event. It provides a mathematical formula for determining conditional probability. Conditional probability is the likelihood of an outcome occurring, based on a previous outcome having occurred in similar circumstances.

The process of updating beliefs in light of new evidence is known as Bayesian inference. This approach is fundamentally different from the frequentist perspective, which defines probability strictly in terms of the frequency of events occurring in the long run. Bayesian methods can work effectively with smaller samples by incorporating prior knowledge, which can be particularly useful in fields where data is scarce or expensive to obtain.

Applications of Bayesian probability and Bayes' theorem are widespread and not limited to the financial realm. For example, they can be used to determine the accuracy of medical test results, rate the risk of lending money to potential borrowers, and in various fields of science, engineering, medicine, and economics.

## How is bayesian probability used in machine learning?

Bayesian probability is used in machine learning in several ways, primarily to model and reason about uncertainty. It provides a principled way for calculating a conditional probability, which is useful in various machine learning applications.

One of the key uses of Bayesian probability in machine learning is in the creation of Bayesian classifiers, such as the Naive Bayes Classifier. These classifiers use Bayes' Theorem to predict the class of given data points.

Bayesian probability is also used in Bayesian optimization, which is a sequential design strategy for global optimization of black-box functions that takes into account uncertainty.

Another application is in Bayesian networks or Bayesian Belief Networks, which are probabilistic graphical models that represent a set of variables and their conditional dependencies via a directed acyclic graph.

Bayesian probability is also used in the Maximum A Posteriori (MAP) estimation, which is a method of estimating the parameters of a statistical model. It uses Bayes' Theorem to update the probability of a hypothesis as more evidence or information becomes available.

In the Bayesian framework for machine learning, you start by enumerating all reasonable models of the data and assigning your prior belief to each of these models. Then, upon observing the data, you evaluate how probable the data was under each of these models to compute the likelihood.

Bayesian inference is a learning technique that uses probabilities to define and reason about our beliefs. It combines explicit prior knowledge with the data to deduce the posterior distribution of models given the data. This approach is particularly useful when data are sparse and the model is dense.

However, implementing Bayesian inference in machine learning often involves the use of computational techniques such as Markov chain Monte Carlo (MCMC) methods and variational inference. These techniques are used to approximate the posterior distribution, which is often intractable in complex models.

Bayesian probability provides a robust framework for modeling and reasoning about uncertainty, making it a powerful tool in machine learning. It allows for the incorporation of prior knowledge, handles small data well, and provides a principled approach to learning from data.

## What are some real-world applications of bayesian probability?

Bayesian probability has a wide range of real-world applications across various fields:

1. Healthcare and Medicine — Bayesian probability is used in medical testing to estimate the probability of a patient having a condition given a positive test result. It can also be used to assess the probability of a disease based on factors such as a person's age.

2. Finance — In finance, Bayes' Theorem can be used to revise existing predictions or theories given new or additional evidence. It can also be used to rate the risk of lending money to individuals or businesses.

3. Machine Learning — Bayesian probability plays a crucial role in machine learning, where it's used to formalize the process of reasoning with uncertain information. This allows computers to make good decisions even when they're not totally sure about certain facts.

4. Genetics — Bayesian inference is applied in genetics, where it can be used to model and understand the complex network of gene interactions.

5. Image Processing — Bayesian methods are used in image processing to make sense of visual data, improve image quality, and perform object recognition.

6. Linguistics — In linguistics, Bayesian probability is used to model language learning and comprehension processes.

7. Search and Rescue Operations — Bayesian search theory, an application of Bayesian statistics, has been used in search and rescue operations to locate lost objects or persons.

8. Decision Making in Everyday Life — Bayesian analysis can be applied to everyday life situations, such as dating and friendships. By examining past experiences, one can modify actions or behaviors to achieve desired outcomes.

These applications demonstrate the versatility and practicality of Bayesian probability in dealing with uncertainty and making informed decisions based on available evidence.

## More terms

### Semantics

Semantics in AI refers to the study and understanding of the meaning of words and phrases in a language. It involves the interpretation of natural language to extract the underlying concepts, ideas, and relationships. Semantics plays a crucial role in various AI applications such as natural language processing, information retrieval, and knowledge representation.

### What is brute-force search?

Brute-force search, also known as exhaustive search or generate and test, is a general problem-solving technique and algorithmic paradigm that systematically enumerates all possible candidates for a solution and checks each one for validity. This approach is straightforward and relies on sheer computing power to solve a problem.