AI Techniques and Methods
Artificial Intelligence

AI Techniques and Methods

Mandi D 8 September 2023

Artificial intelligence as a field encompasses a vast range of programming techniques, architectures and methods that aim to tackle challenges in reasoning, knowledge representation, search, optimization and generative modeling.

While machine learning and neural networks represent some of the most popularized approaches in recent years, the toolkit of AI is far more diverse. Mastering fundamental techniques across reasoning, search, logic and beyond enables creating systems that can replicate and augment human cognition and abilities. This article provides an overview of established and emerging methods propelling AI capabilities.

Reasoning and Knowledge Representation

Endowing machines with the capacity for logic, reason and rational decision-making akin to humans has been a central pursuit since AI’s origins. Rather than simply following programmed instructions, AI systems need flexible representations of knowledge along with mechanisms for applying logic and drawing inferences about that knowledge. Key approaches in this area include:

  • Symbolic logic and reasoning: Applying the rigor of mathematical formal logic to let AIs reason through problems in symbolic representations. Common techniques involve propositional logic, first-order logic, inference using rules like modus ponens, logical deduction using theorem proving, and more. Knowledge is encoded in symbolic representations which the AI manipulates through valid logic operations.
  • Knowledge graphs: Representing concepts and entities as nodes in a graph, with relationships between them denoted as links. Enables systems to reason about the properties of objects and how they relate. Knowledge graphs now enable many AI applications from search engines to recommendations.
  • Ontologies: Conceptual models that formalize the objects, classes, attributes and relations in a domain of knowledge or discourse. Shareable ontologies allow standardization of knowledge representation across systems and users. Enable advanced inference and reasoning through hierarchical classifications, properties and relations between concepts.
  • Rules and constraints: Constraint satisfaction problems define constraints and logical rules that acceptable solutions must satisfy. Used heavily in expert systems applied to specialized domains like medical diagnosis. Constraint programming languages also allow users to efficiently describe complex systems of rules and constraints.

Altogether, these techniques equip AI agents with structured knowledge about the world which they can then analyze with logic mechanisms to reach conclusions and act rationally. Just as humans leverage experience and facts encoded in memory, combined with innate reasoning abilities, AI systems require robust knowledge representations and reasoning skills. Recent research has built large commonsense knowledge bases and reasoning engines to power more human-like inferences and decision making.

Search Algorithms

Many complex AI problems require navigating vast spaces of possible states and solutions to find optimal or sufficiently good ones. Games like chess have over 10^120 possible move configurations, while tasks like determining 3D molecular protein folding involve enormous search spaces. Solving such problems involves employing smart search algorithms and heuristics. Key techniques include:

  • Informed search: Use domain heuristics and estimated cost/distance metrics to guide which states or nodes to explore next in search. Greatly improves efficiency over uninformed brute force search. Allows expanding the most promising node according to specified rules. Common algorithms like A* efficiently find shortest paths this way.
  • Optimization algorithms: Iteratively refine candidate solutions to minimize an objective cost function. Allows traversing large solution spaces to find global or local optima. Includes popular techniques like gradient descent, Newton’s method, linear and quadratic programming, and more. Used heavily in machine learning for finding neural network weights.
  • Sampling methods: Assess a smaller subset of possible solutions instead of exhaustively checking all combinations. Probabilistic methods like Markov Chain Monte Carlo and random sampling allow estimating properties of very large state spaces. Used commonly in simulations and games.
  • Metaheuristics: High-level strategies or “rules of thumb” to guide an underlying search or optimization process, like evolutionary algorithms and tabu search. Can overcome limitations of narrow gradients or local optima in complex search landscapes.

Better search algorithms allow AI agents to efficiently reason about massive state and solution spaces, optimizing decisions by approximating global insight from local exploration. Techniques continue advancing to expand the scale of computable problems, such as through heuristics specialized to certain domains.

Logical Reasoning and Theorem Proving

Formal logic provides a rigorous mathematical foundation for deriving valid conclusions through sound chains of deductive reasoning. Symbolic logic has long been studied in AI to automate logical reasoning and theorem proving. Major approaches include:

  • Propositional logic: Logical propositions described with Boolean variables, logical connectives (AND, OR, NOT) and equivalences. Allows proving the validity of argument forms and logical deductions using truth tables.
  • First-order logic: Includes quantifiers over objects like “for all” and “there exists”, along with predicates relating objects. More expressive for describing real-world facts and inferring new information through deduction.
  • Theorem proving: Using axioms and inference rules to mathematically prove theorems in domains like geometry, algebra and logic itself. Mechanizing the process allows deriving new theorems. Implemented in systems like Coq, Mizar, etc.
  • Logic programming: Declarative programming languages like Prolog based in first-order logic. Describes relations between objects and queries facts in logical clauses, enabling logic-based computation.

Advancing logical reasoning capabilities allows AI systems to prove mathematical theorems, infer new facts from existing knowledge, answer questions, and make trustworthy deductions free of human cognitive biases. While logic alone cannot replicate common sense or contextual reasoning, combining formal logic with probabilistic and neuromorphic techniques remains an active research pursuit.

Evolutionary Algorithms

Evolutionary algorithms draw inspiration from Darwinian principles of natural selection to iteratively develop optimal or highly fit solutions. Variants implement digital analogues of mutation, inheritance, crossover, and selection over successive generations to breed better solutions without requiring domain-specific heuristics. Common techniques include:

  • Genetic algorithms: Encodes candidate solutions as a genome or chromosome. Creates subsequent generations by combining and randomly mutating chromosomes based on their fitness, enabling evolution. Used for optimization, search, and machine learning.
  • Genetic programming: Evolves full computer programs or algorithms as part of the chromosomes based on fitness objectives like accuracy or efficiency. May develop novel algorithms and optimizations.
  • Swarm intelligence: Decentralized collective intelligence emerging from large swarms of simple agents following simple rules, influenced by insect colonies, flocks of birds, etc. Exhibits intelligent group behavior despite limited individual capabilities.
  • Ant colony optimization: Virtual ant colonies develop optimal paths by depositing pheromones as they traverse graphs. Positive feedback leads to dense pheromone trails on efficient routes. Used for routing, scheduling, protein folding, and more.

Inspired by nature, evolutionary algorithms are general-purpose tools effective for hard optimization problems, capable of escaping local optima unlike greedy methods. Continued research looks to improve performance, incorporate learned knowledge, and apply evolutionary principles to new domains like neural architecture search.

Statistical Learning and Pattern Recognition

Identifying patterns is a core capability underlying tasks from image recognition to market forecasting. Statistical learning techniques apply probability, statistics and optimization to make predictions by generalizing from data examples. Main methods include:

  • Supervised learning: Models are trained on labeled example inputs and outputs, learning a mapping between them. Goal is predicting the right outputs for new inputs. Includes algorithms like logistic regression and support vector machines.
  • Unsupervised learning: Finds hidden patterns in unlabeled data lacking defined outputs. Used for tasks like clustering, dimensionality reduction, and density estimation. Algorithms include k-means, principal component analysis, mixture models, and autoencoders.
  • Semi-supervised learning: Combines a small labeled dataset with a larger unlabeled dataset during training. Can reduce labeling efforts while taking advantage of unlabeled patterns. Includes techniques like label propagation over graphs.
  • Transfer learning: Leverages model knowledge gained from solving one problem to benefit related problems with little data. May fine-tune pretrained models rather than train from scratch. Enables better performance with limited task-specific data.
  • Multitask learning: Jointly trains model on multiple problems to exploit commonalities and differences between tasks. Improves generalization via shared representations.
  • Active learning: Incrementally builds training sets by intelligently sampling most informative unlabeled data points for labeling by oracle, reducing labeling costs.

Altogether, statistical learning delivers powerful, general-purpose tools for recognizing patterns, predicting outcomes and making data-driven decisions based on historical examples. Recent breakthroughs in deep neural networks have enabled unmatched skill at perceptual tasks involving images, text, voice and video.

Reinforcement Learning

Reinforcement learning tackles sequential decision making problems through trial-and-error interactions with an environment. Agents learn by maximizing rewards and minimizing penalties associated with state actions, optimizing a policy for choosing actions. Key methods include:

  • Markov decision processes: Mathematical framework representing an environment with states, actions, transitions and rewards. Allows computation of optimal policies.
  • Q-learning: Estimates expected rewards of state-action pairs iteratively based on experience. Enables learning without a model of environment dynamics.
  • Policy gradients: Optimizes parameterized policies by gradient ascent on expected rewards. Allows direct policy search rather than indirect action-value estimates.
  • Actor-critic methods: Combine value estimation for optimal actions with policy optimization. More efficient by sharing representations.
  • Inverse reinforcement learning: Infer the reward function optimized by another agent from its behavior. Enables imitation learning and preference modeling.

Reinforcement learning combinesessential abilities of goal-oriented behavior, real-time decision-making, and learning from experience. It has enabled advances in robotics, game-playing, resource management, and other applications requiring complex, adaptive behavior without explicit supervision.

Probabilistic Graphical Models

Probabilistic graphical models compactly represent complex probability distributions over multiple interacting variables using graphs. Nodes denote variables, edges denote dependencies. Key advantages include efficient inference, ability to handle uncertainty, and interpreting predictive models. Types include:

  • Bayesian networks: Directed graphical models where nodes represent random variables and edges encode conditional dependencies via parent-child relationships. Supports efficient probabilistic inference by factorizing joint distributions.
  • Markov networks: Undirected graphical models representing dependencies between random variables as neighborhood relationships. Include Markov random fields and conditional random fields popular for spatial and sequence modeling.
  • Decision networks: Graphical models that encode conditional probability distributions together with utilities/rewards to describe outcomes of actions. Used for decision analysis under uncertainty.

Probabilistic graphical models provide an intuitive yet powerful framework for reasoning about uncertainty and causal relationships in real-world data. They enable explaining model predictions, incorporating domain expertise, and making decisions under uncertainty across applications in science, biomedicine, engineering and more.

Natural Language Processing

Processing and understanding human languages is crucial for tasks like text summarization, dialogue systems, and question answering. Main NLP techniques include:

  • Information retrieval: Finding relevant texts or documents matching an input query or user intent. Involves indexing, storage, search, ranking of documents.
  • Information extraction: Identifying structured information like entities, relationships, facts from unstructured text using parsing and machine learning.
  • Machine translation: Automating translation between human languages using statistical and neural models. Allows cross-lingual information access.
  • Dialogue systems: Automated conversational agents able to understand requests, ask questions, offer recommendations via speech or text.
  • Summarization: Automatically condensing longer text into concise summaries capturing key ideas, context, tone. Requires natural language understanding.
  • Question answering: Systems that infer specific answers to natural language questions in domains like medicine, finance, law. Rely on information retrieval, knowledge bases, reasoning.

Ongoing advances in representing semantics, context, vocabulary, and syntax continue to expand machines’ linguistic capabilities, with applications across domains that depend on communicating in human languages.

Generative Modeling

Generative models learn representations of complex high-dimensional data distributions, allowing sampling or reconstructing new datapoints from the learned distribution. Key approaches include:

  • Neural networks: Architectures like generative adversarial networks, variational autoencoders and flow-based models effectively generate images, speech, text, molecular structures, and other modalities.
  • Autoregressive models: Sample data sequentially one step at a time, with the next step conditioned on previous ones. Includes PixelCNN for images and GPT models for text generation.
  • Energy-based models: Learn data densities by assigning low energies to preferred inputs and higher energies to unlikely ones. Allows directly modeling complex multimodal distributions.
  • Diffusion models: Iteratively add noise and then remove it as a Markov chain process, providing control over generated samples. Includes DDPM and score-based diffusion models.

Generative modeling enables creating novel, realistic samples from complex domains without manual effort. Applications range from content creation to drug discovery, finite element analysis, 3D scene modeling and beyond.

The techniques highlighted in above represent a cross-section of the extensive toolkit driving AI’s capabilities. Advances in core disciplines like search, logic, knowledge representation, planning and statistics supply fundamental building blocks for increasingly performant and general-purpose AI systems.

Integrating multiple algorithms tailored to the problem structure allows tackling real-world complexity. The future pace of AI progress will continue relying not just on individual techniques, but on combining complementary approaches in novel architectures and workflows.