AI

Conditional Probability & Bayes’ Theorem

Conditional Probability & Bayes’ Theorem #

Probability often changes when we learn new information.

Conditional probability and Bayes’ theorem give a structured way to update beliefs using evidence.

Conditional probability updates probabilities after observing an event.

Bayes’ theorem lets you estimate a hidden cause from observed evidence.

Naïve Bayes turns Bayes’ theorem into a practical classifier by assuming conditional independence of features given the class.


flowchart TD

A[Conditional<br/>probability] -->|foundation| B[Bayes<br/>theorem]
D[Independent<br/>events] -->|implies| C[Independence]
C -->|simplifies| A

E[Prior] -->|with likelihood| B
F[Likelihood] -->|updates| H[Posterior]
G[Evidence] -->|normalises| B
B -->|yields| H

I[Naïve<br/>Bayes] -->|uses| B
J[Naïve<br/>assumption] -->|assumes| C
K[Features] -->|given class| J
L[Class] -->|conditions| J
I -->|predicts| M[Classification]
M -->|selects| L

style A fill:#90CAF9,stroke:#1E88E5,color:#000
style B fill:#90CAF9,stroke:#1E88E5,color:#000
style C fill:#90CAF9,stroke:#1E88E5,color:#000

style D fill:#CE93D8,stroke:#8E24AA,color:#000
style E fill:#CE93D8,stroke:#8E24AA,color:#000
style F fill:#CE93D8,stroke:#8E24AA,color:#000
style G fill:#CE93D8,stroke:#8E24AA,color:#000
style J fill:#CE93D8,stroke:#8E24AA,color:#000
style K fill:#CE93D8,stroke:#8E24AA,color:#000
style L fill:#CE93D8,stroke:#8E24AA,color:#000

style H fill:#C8E6C9,stroke:#2E7D32,color:#000
style I fill:#C8E6C9,stroke:#2E7D32,color:#000
style M fill:#C8E6C9,stroke:#2E7D32,color:#000


Quick summary #

  • Conditional probability: updates probability after an event is known.
  • Multiplication rule: computes joint probability from conditional parts.
  • Independence: tested using \( P(A\cap B)=P(A)P(B) \) .
  • Total probability: breaks a probability into weighted cases.
  • Bayes’ theorem: reverses conditioning to infer causes from evidence.

What’s next #

Probability Distributions
Move from events to random variables and distributions.

AI Stack

AI Stack #

The AI Stack describes the layers required to build an end-to-end AI system, from infrastructure at the bottom to user-facing applications at the top.

Different organisations represent the AI stack differently; this is a simplified conceptual view for learning.

Each layer depends on the one below it.


graph TB

    subgraph APP["Applications"]
        A[User Interfaces & Integrations]
    end

    subgraph ORCH["Orchestration"]
        O[Workflows • Agents • Control Logic]
    end

    subgraph DATA["Data"]
        D[Data Sources • Pipelines • Vector DBs]
    end

    subgraph MODEL["Models"]
        M[ML • DL • Foundation Models • LLMs]
    end

    subgraph INFRA["Infrastructure"]
        I[Cloud • On-prem • GPUs • Storage]
    end

    %% Styling
    style APP fill:#FFCCBC
    style ORCH fill:#90CAF9
    style DATA fill:#BBDEFB
    style MODEL fill:#C8E6C9
    style INFRA fill:#E1F5FE

    style A fill:#FFE0B2
    style O fill:#B3E5FC
    style D fill:#E3F2FD
    style M fill:#DCEDC8
    style I fill:#E1F5FE

1. Infrastructure #

The foundation that provides compute and storage.

Artificial Neuron and Perceptron

Artificial Neuron and Perceptron #

knowledge in neural networks is stored in connection weights, and learning means modifying those weights.


Biological Neuron #

A biological neuron is a specialised cell that processes and transmits information through electrical and chemical signals.

Core components:

  • Dendrites: receive signals from other neurons
  • Cell body (soma): processes incoming signals
  • Axon: transmits the output signal
  • Synapses: connection points between neurons

Biological intuition:

  • many inputs arrive to one neuron
  • one neuron can connect out to many neurons
  • massive parallelism enables fast perception and recognition

Artificial Neuron #

An artificial neuron is a simplified computational model inspired by biological neurons.

Intelligent Agents and Environments

Intelligent Agents and Environments #

An intelligent agent connects perception with action. It observes an environment through sensors, uses the information it receives to decide what to do, and affects the environment through actuators.

Develop the core vocabulary for reasoning about intelligent agents: percepts, actions, rationality, performance measures, PEAS and the different properties an environment can have.

Learning Objectives #

  • explain the relationship between an agent and its environment
  • distinguish sensors, percepts, actions and actuators
  • explain percept sequences and the agent function
  • describe what makes an agent rational
  • distinguish rationality from omniscience
  • specify a task environment using PEAS
  • classify environments using standard AI properties
  • explain why environment properties affect agent design

Big Picture #

flowchart TD
    E[Environment] -->|Percepts| S[Sensors]
    S --> A[Agent]
    A --> C[Actuators]
    C -->|Actions| E

    style E fill:#E1F5FE
    style S fill:#FFF9C4
    style A fill:#C8E6C9
    style C fill:#EDE7F6

Think of an agent as a continuous loop: observe → decide → act → observe again. An action may change the environment, so the next observation may be different.

ML Workflow

Machine learning Workflow #

Data is the foundation of any machine learning system. Quality of data matters more than model complexity.

Role of Data #

Data determines:

  • What patterns the model can learn
  • How well it generalises
  • Whether bias or noise is introduced

Bad data → bad model (even with perfect algorithms).


Data Preprocessing, wrangling #

Raw data is never ready for training.

Data Issues

  • Noise
    • For objects, noise is an extraneous object
    • For attributes, noise refers to modification of original values
    • Use Log or Z Transfer to convert to mean
  • Outliers
    • Data objects with characteristics that are considerably different than most of the other data objects in the data set
    • Handle: Use IQR method
    • Find Lower and Upper Bound and replace Outlier with Lower or Upper Bound
  • Missing Values
    • Eliminate data objects or variables
    • Handle: Estimate missing values
      • Mean, Median or Mode
      • Prefer Median if there are missing outliers
    • Ignore the missing value during analysis
  • Duplicate Data
    • Major issue when merging data from heterogeneous sources
  • Inconsistent Codes
    • Find all Unique and transfer all inconsistent to

Data Preprocessing techniques

Multi-Armed Bandit Problem

Multi-Armed Bandit Problem #

The Multi-Armed Bandit (MAB) problem is the simplest setting for studying decision-making under uncertainty.

An agent repeatedly chooses one of \( k \) actions. Each action produces a numerical reward drawn from an unknown distribution. The objective is to maximise the expected total reward over time.

The central challenge is deciding when to exploit current knowledge and when to explore uncertain alternatives.

The k-Armed Bandit Problem ☆ #

At every time step:

Parallel and Distributed Algorithms

Parallel and Distributed Algorithms #

Parallelisation divides computational work into parts that can execute concurrently. The purpose is to reduce completion time or increase throughput, but the gain depends on how much work is genuinely independent and how much overhead is introduced.

This page covers:

  • speedup, maximum speedup, and processor efficiency
  • Amdahl’s Law
  • data-level parallelism
  • task-level parallelism
  • algorithm-specific parallelism
  • communication, synchronisation, scheduling, and load-balancing overhead
  • parallel merge sort and matrix multiplication

Learning Objectives #

By the end of this page, you should be able to:

Vector Semantics and Embedding

NLP - Vector Semantics and Embedding #

  • Lexical semantics and word meaning.
  • Lemmas, senses, and semantic relationships.
  • Distributional hypothesis.
  • Vector semantics and word embeddings.
  • Document and word vectors.
  • Dot product and cosine similarity.
  • Term Frequency–Inverse Document Frequency (TF-IDF).
  • Prediction-based word embeddings and self-supervision.
  • Word2Vec using Skip-gram with Negative Sampling and CBOW.
  • Embedding matrices, context-window choices, analogies, visualisation, and bias.
  • GloVe and global word–word co-occurrence statistics.

Learning Objectives #

  • Explain lexical semantics and distinguish a lemma from a word sense.
  • Compare synonymy, similarity, relatedness, antonymy, and connotation.
  • Explain the distributional hypothesis and its role in modelling meaning.
  • Describe how words and documents can be represented as vectors.
  • Construct and interpret word–document and word–context matrices.
  • Calculate dot product and cosine similarity between vectors.
  • Explain why raw word frequency can be misleading.
  • Calculate TF, IDF, and TF-IDF weights.
  • Explain how Word2Vec learns embeddings from a prediction task.
  • Construct positive and negative Skip-gram training pairs.
  • Explain how sigmoid, negative sampling, and gradient descent train SGNS.
  • Compare Skip-gram with CBOW.
  • Explain how context-window size affects the relationships captured.
  • Interpret word analogies and two-dimensional embedding visualisations.
  • Explain how GloVe combines global counts with learned dense vectors.
  • Recognise how social biases can be encoded in word embeddings.

Big Picture #

flowchart TD
    A[Words and Documents] --> B[Observe Their Context]
    B --> C[Represent Them as Vectors]
    C --> D[Compare Vector Directions]
    D --> E[Estimate Semantic Similarity]
    E --> F[Search, Classify, Retrieve or Generate]

    style A fill:#E1F5FE
    style B fill:#C8E6C9
    style C fill:#FFF9C4
    style D fill:#EDE7F6
    style E fill:#E1F5FE
    style F fill:#C8E6C9

1. Lexical Semantics ☆ #

Lexical semantics is the linguistic study of word meaning and the relationships between word meanings.

Conditional Probability

Conditional Probability #

Conditional probability updates the probability of an event when new information is available.

It shows up whenever a question says:

  • “given that…”
  • “among those who…”
  • “out of the items that…”
  • “if it does not fail immediately…”

Key takeaway: Conditional probability is always:

joint probability ÷ probability of the condition.

The condition must not be an impossible event.


Prior vs posterior #

  • Prior probability: probability with no condition (before new information)

Bayes’ Theorem

Bayes’ Theorem #

2.1 Total probability (needed for Bayes) #

Often we split the world into cases \( E_1,E_2,\dots,E_k \) that:

  • are mutually exclusive
  • cover the whole sample space

Then for any event \( A \) :

\[ P(A)=\sum_{i=1}^{k} P(A\mid E_i)\,P(E_i) \]

Tree intuition: