Probability

Introduction to Probability

A first course in probability, from events, counting and conditional probability to random variables, discrete and continuous distributions, limit theorems, simulation and Markov chains.

  • Introductory
  • 84 lessons
  • Free

About this course

Probability is the mathematics of uncertainty. A probability model describes what can happen and assigns probabilities that obey a few rules, and those rules underlie statistics, simulation and the analysis of risk. This course starts from events and the probability axioms and works through counting, conditional probability and Bayes’ theorem, discrete random variables and their standard distributions, joint distributions, conditional expectation and a law of large numbers for averages. It then introduces the calculus needed for continuous distributions and continues with the exponential and normal models, the central limit theorem, simulation, random walks, Poisson processes and small Markov chains.

The videos are coming out one at a time, in course order. Every lesson has its own page with precise definitions, a worked example for each key term and a few exercises with answers, so you can start reading now.

The reference sheet collects the definitions, rules, distributions and theorems unit by unit, with the conditions each one needs and links to the lessons that explain them.

No calculus is needed to start. Fractions, percentages and elementary algebra are enough for the early lessons, and before continuous distributions the course teaches the basic integrals and derivatives they need, as a short bridge rather than a full calculus course. A few later lessons also rely on the exponential function and on matrix multiplication. In Precalculus, the Exponential and Logarithmic Functions unit covers the functions behind the Poisson and exponential distributions, and the Systems, Vectors, and Matrices unit covers matrix multiplication. Calculus develops integrals and derivatives fully, including a lesson on integration in probability theory.

Statistics teaches the probability it needs on its own, so the two courses can be taken in either order. Its lessons on sampling distributions, standard errors, confidence intervals, tests and Bayesian updating use ideas developed here, and the lesson pages of this course point to them. Multivariate Statistical Analysis lists probability and random variables among its prerequisites, and the joint distributions, covariance and correlation from this course return there as random vectors and covariance matrices.

Good to know first

  • Fractions and percentages
  • Elementary algebra
  • Reading graphs and tables

Course outline

1 of 84 lessons are on YouTube so far. Each lesson is one video. Open a unit to see its lessons, or follow a lesson for its definitions and examples.

  1. Probability Foundations

    1 of 8 lessons on YouTube
    Open the Probability Foundations unit page

    Probability Models and Events

    1. Video 1

      experiment, trial, outcome, event, probability model, theoretical probability, empirical relative frequency and subjective probability

      Also uses fractions and percentages, elementary algebra and reading tables and graphs.

    2. Video 2

      sample space, union, intersection, complement, set difference, De Morgan’s laws, disjoint events and partition

      Builds on Probability Models and Uncertainty.

    3. Video 3

      inclusive or, neither, not both, exactly one, at least, at most, only if and counterexample

      Builds on Sample Spaces and Events.

    The Axioms and Basic Rules

    1. Video 4

      probability axioms, nonnegativity, normalization, countable additivity, pairwise disjoint events, probability of the empty event and monotonicity

      Builds on Sample Spaces and Events.

    2. Video 5

      complement rule, complementary partition, at least one, not all and complement of at least k

      Builds on Probability Axioms and Basic Rules and Translating Event Language.

    3. Video 6

      addition rule, overlap, disjoint regions, exactly one of two events and union bound

      Builds on Probability Axioms and Basic Rules and Translating Event Language.

    Equally Likely Outcomes and Simulation

    1. Video 7

      equally likely outcomes, uniform finite model, favorable outcomes, cardinality, level of detail and ordered sequences

      Builds on Probability Axioms and Basic Rules, The Complement Rule and The Addition Rule.

    2. Video 8

      simulation, simulated trial, pseudorandom generator, random seed, reproducibility, running estimate and sampling variability

      Builds on Equally Likely Outcomes.

  2. Counting and Equally Likely Models

    7 lessons coming soon
    Open the Counting and Equally Likely Models unit page

    Ordered and Unordered Selections

    1. Video 9

      product rule for counting, sum rule for counting, counting by cases, selection with replacement, selection without replacement, counting with restrictions and decision tree

      Builds on Equally Likely Outcomes.

    2. PermutationsComing soon

      Video 10

      factorial, zero factorial, permutation, partial permutation and circular arrangement

      Builds on Sum and Product Rules for Counting.

    3. Video 11

      combination, binomial coefficient, overcounting, symmetry of binomial coefficients, Pascal’s identity, Pascal’s triangle and binomial theorem

      Builds on Permutations.

    4. Video 12

      multiset permutation, indistinguishable objects, multinomial coefficient, counting by choosing positions, allocation to named groups and multinomial expansion

      Builds on Combinations and Binomial Coefficients.

    Overlaps and Collisions

    1. Video 13

      inclusion-exclusion principle, pairwise intersection, triple intersection, disjoint Venn regions, exactly one of three events and exactly two of three events

      Builds on The Addition Rule and Combinations and Binomial Coefficients.

    2. Video 14

      collision, no-collision probability, birthday problem, match of a specified pair, pigeonhole principle and nonuniform categories

      Builds on Inclusion-Exclusion and The Complement Rule.

    Choosing a Counting Method

    1. Video 15

      ordered selection, unordered selection, unordered selection with repetition, multiplicity of a record, equal likelihood after erasing order and counting by complement

      Builds on Sum and Product Rules for Counting, Permutations, Combinations and Binomial Coefficients, Repeated Objects and Multinomial Counting, Inclusion-Exclusion and Birthday and Collision Problems.

  3. Conditioning, Independence and Evidence

    9 lessons coming soon
    Open the Conditioning, Independence and Evidence unit page

    Conditional Probability

    1. Video 16

      conditional probability, conditioning event, reference group, reversed condition and positive-probability requirement

      Builds on Probability Axioms and Basic Rules and The Addition Rule.

    2. Video 17

      two-way table, joint probability, marginal probability, margins, row and column percentages and natural frequencies

      Builds on Conditional Probability.

    Independence

    1. Video 18

      independent events, product criterion, dependent events, independence of complements, pairwise independence and mutual independence

      Builds on Conditional Probability.

    2. Video 19

      disjoint events with positive probability, overlapping independent events, overlapping dependent events, zero-probability event and nested events

      Builds on Independent Events and The Addition Rule.

    Trees, Total Probability and Bayes’ Theorem

    1. Video 20

      probability tree, conditional branch, multiplication rule, chain rule, terminal path and branch normalization

      Builds on Sum and Product Rules for Counting, Conditional Probability and Independent Events.

    2. Video 21

      law of total probability, group weight, conditional rate, weighted average and zero-weight group

      Builds on Conditional Probability and Probability Trees and the Chain Rule.

    3. Video 22

      Bayes’ theorem, prior probability, likelihood, posterior probability, base rate neglect, sensitivity, specificity and false positive rate

      Builds on The Law of Total Probability.

    Combining Evidence

    1. Video 23

      conditional independence, latent state, shared cause, marginal dependence, sequential updating and duplicated evidence

      Builds on Bayes’ Theorem and Base Rates and Independent Events.

    2. Video 24

      odds in favor, odds against, probability from odds, likelihood ratio, prior odds, posterior odds and odds form of Bayes’ theorem

      Builds on Bayes’ Theorem and Base Rates and Conditional Independence and Sequential Evidence.

  4. Discrete Random Variables and Expected Value

    6 lessons coming soon
    Open the Discrete Random Variables and Expected Value unit page

    Random Variables and Their Distributions

    1. Video 25

      random variable, realization, discrete random variable, support, countably infinite support and distribution of a random variable

      Builds on Equally Likely Outcomes, Conditional Probability and Independent Events.

    2. Video 26

      probability mass function, nonnegative masses, normalizing constant, event probability as a sum of masses, finite uniform distribution and probability bar chart

      Builds on Discrete Random Variables.

    3. Video 27

      cumulative distribution function, step function, jump size, right-continuity, tail probability and interval probability

      Builds on Probability Mass Functions.

    4. Video 28

      function of a random variable, preimage, one-to-one map, many-to-one map, collecting probability mass and positive part

      Builds on Probability Mass Functions and Cumulative Distribution Functions.

    Expected Value

    1. Expected ValueComing soon

      Video 29

      expected value, probability-weighted average, mode, conditional mean, net gain, infinite expectation and absolute integrability

      Builds on Discrete Random Variables and Probability Mass Functions.

    2. Video 30

      expected payoff, net payoff, fair price, break-even probability, expected cost, model risk and decision criterion

      Builds on Expected Value.

  5. Expectation, Spread, Sums and Averages

    6 lessons coming soon
    Open the Expectation, Spread, Sums and Averages unit page

    Linearity and Indicators

    1. Video 31

      linearity of expectation, expectation of a sum, linear combination, expectation of a product, expectation of a square and finite expectation

      Builds on Expected Value and Functions of Discrete Random Variables.

    2. Video 32

      indicator random variable, expectation of an indicator, sum of indicators, expected count, indicator of a union and expected number of distinct categories

      Builds on Linearity of Expectation and Discrete Random Variables.

    Variance and Standard Deviation

    1. VarianceComing soon

      Video 33

      variance, deviation from the mean, squared deviation, second moment, computational formula for variance, squared units and finite second moment

      Builds on Expected Value, Linearity of Expectation and Functions of Discrete Random Variables.

    2. Video 34

      standard deviation, root-mean-square deviation, mean absolute deviation, shift, change of scale, affine transformation and change of units

      Builds on Variance.

    Sums and Sample Means

    1. Video 35

      distribution of a sum, ordered-pair table, discrete convolution, variance of an independent sum and cross term

      Builds on Independent Events, Probability Mass Functions, Linearity of Expectation and Variance.

    2. Video 36

      random sample, sample mean, statistic, sampling distribution, independent and identically distributed and standard error

      Builds on Sums of Discrete Random Variables and Cumulative Distribution Functions.

  6. Discrete Probability Distributions

    7 lessons coming soon
    Open the Discrete Probability Distributions unit page

    Uniform and Bernoulli Distributions

    1. Video 37

      discrete uniform distribution, number of support values, mean by symmetry, variance of consecutive integers, sum-of-squares identity and evenly spaced support

      Builds on Probability Mass Functions, Expected Value and Variance. Also uses the sum of the first n positive integers.

    2. Video 38

      Bernoulli distribution, Bernoulli trial, success, parameter of a distribution, mean and variance of a Bernoulli variable, degenerate distribution and maximum Bernoulli variance

      Builds on Probability Mass Functions, Expected Value and Variance.

    Counting and Waiting in Independent Trials

    1. Video 39

      binomial distribution, binomial probability formula, binomial assumptions, mean of a binomial count, variance of a binomial count and binomial tail probability

      Builds on The Bernoulli Distribution, Combinations and Binomial Coefficients, Variance, Sums of Discrete Random Variables and Indicator Random Variables.

    2. Video 40

      geometric distribution, trial-count convention, failure-count convention, geometric series, geometric tail, memoryless property and mean and variance of a geometric distribution

      Builds on The Bernoulli Distribution and The Binomial Distribution.

    3. Video 41

      negative binomial distribution, stopping rule, waiting time for the rth success, failure-count form, sum of geometric waiting times and deadline probability as a binomial tail

      Builds on The Geometric Distribution, The Binomial Distribution and Combinations and Binomial Coefficients.

    Finite Populations and Poisson Counts

    1. Video 42

      hypergeometric distribution, finite population, sampling without replacement, support of a hypergeometric count, finite population correction and dependent draws

      Builds on Combinations and Binomial Coefficients, Probability Trees and the Chain Rule and The Binomial Distribution.

    2. Video 43

      Poisson distribution, Poisson parameter, rate times exposure, exponential series, Poisson mean and variance and Poisson approximation to the binomial

      Builds on The Binomial Distribution, Expected Value and Variance. Also uses the exponential function and the number e.

  7. Joint Discrete Laws and Conditional Averages

    8 lessons coming soon
    Open the Joint Discrete Laws and Conditional Averages unit page

    Joint, Marginal and Conditional Distributions

    1. Video 44

      random vector, joint probability mass function, joint probability table, joint support, event region and function of a random pair

      Builds on Probability Mass Functions and Conditional Probability.

    2. Video 45

      marginal distribution, marginal probability mass function, summing out a variable and information lost by marginalizing

      Builds on Joint Probability Mass Functions.

    3. Video 46

      conditional probability mass function, renormalizing a row or column, reversed conditioning, conditional mean from a conditional PMF and joint probability from conditional and marginal

      Builds on Marginal Probability Distributions, Conditional Probability and Expected Value.

    Independence and Covariance

    1. Video 47

      independent random variables, product law, joint cumulative distribution function, CDF factorization, functions of independent variables and mutual independence of random variables

      Builds on Independent Events, Joint Probability Mass Functions, Marginal Probability Distributions, Conditional Probability Mass Functions and Cumulative Distribution Functions.

    2. Video 48

      covariance, correlation coefficient, uncorrelated variables, variance of a sum, bilinearity of covariance, Cauchy-Schwarz inequality and perfect correlation

      Builds on Variance, Independent Random Variables and Joint Probability Mass Functions.

    Conditional Expectation and Total Variance

    1. Video 49

      conditional expectation, conditional mean function, random conditional mean, law of total expectation, integrability and squared-error prediction

      Builds on Expected Value, Linearity of Expectation and Conditional Probability Mass Functions.

    2. Video 50

      conditional variance, law of total variance, within-group variation and between-group variation

      Builds on Variance, Covariance, Correlation and Sums and Conditional Expectation and the Tower Rule.

    3. Video 51

      random sum, random count, empty sum, mean of a random sum and variance of a random sum

      Builds on The Law of Total Variance, Poisson Distributions and Linearity of Expectation.

  8. Repeated Sampling and the Discrete Law of Large Numbers

    6 lessons coming soon
    Open the Repeated Sampling and the Discrete Law of Large Numbers unit page

    Averages and Probability Bounds

    1. Video 52

      realized average, unbiasedness, variance of the sample mean, standard error of the mean and square-root scaling

      Builds on Sample Means and Sampling Distributions, Independent Random Variables and Covariance, Correlation and Sums.

    2. Video 53

      probability bound, Markov’s inequality, Chebyshev’s inequality, sharp bound, two-sided deviation event and bound in standard deviation units

      Builds on Expected Value, Variance and Mean and Variance of the Sample Mean.

    The Law of Large Numbers and Streaks

    1. Video 54

      weak law of large numbers, convergence in probability, error tolerance, relative frequency, sufficient sample size and finite-sample uncertainty

      Builds on Markov and Chebyshev Inequalities, Mean and Variance of the Sample Mean and The Binomial Distribution.

    2. Video 55

      streak, gambler’s fallacy, dilution, compensation misconception, run at a preselected position and learning about an unknown probability

      Builds on Law of Large Numbers for Discrete Variables and Independent Events.

    Simulation Error

    1. Video 56

      simulation estimate, Monte Carlo error, plug-in standard error, conservative standard error, replication, model error and implementation error

      Builds on Probability Simulation, Mean and Variance of the Sample Mean and Law of Large Numbers for Discrete Variables.

    2. Video 57

      repeated sampling, exact benchmark, empirical distribution, empirical variance and replication count

      Builds on Simulation Error and Number of Trials, The Binomial Distribution and Law of Large Numbers for Discrete Variables.

  9. Continuous Probability and Its Mathematical Tools

    6 lessons coming soon
    Open the Continuous Probability and Its Mathematical Tools unit page

    Integrals and Derivatives for Probability

    1. Video 58

      probability density, probability as area, Riemann sum, definite integral, limits of integration, accumulated probability and power rule for integrals

      Builds on Discrete Random Variables, Expected Value and Variance. Also uses areas of rectangles and triangles and an informal idea of a limit.

    2. Video 59

      difference quotient, derivative, tangent slope, power rule for derivatives, fundamental theorem of calculus, antiderivative and density as the derivative of the CDF

      Builds on Integrals and Probability as Area and Cumulative Distribution Functions.

    Densities and Mixed Distributions

    1. Video 60

      probability density function, absolutely continuous distribution, normalization of a density, density units, CDF of a continuous variable, interval probability from a CDF and zero probability at a point

      Builds on Integrals and Probability as Area and Derivatives and the Density-CDF Connection.

    2. Video 61

      point mass, mixed distribution, CDF jump, left limit of a CDF, probability-zero event and almost surely

      Builds on Probability Densities and CDFs and Cumulative Distribution Functions.

    Continuous Moments and Uniform Distributions

    1. Video 62

      expected value of a continuous variable, law of the unconscious statistician, variance of a continuous variable, mean of a mixed distribution, infinite mean and moment existence

      Builds on Integrals and Probability as Area, Derivatives and the Density-CDF Connection, Probability Densities and CDFs and Variance. Also uses integrals over an unbounded interval.

    2. Video 63

      continuous uniform distribution, quantile, percentile, median, mean and variance of a uniform distribution, conditional uniform distribution and change of units for a density

      Builds on Probability Densities and CDFs and Continuous Expectation and Variance.

  10. Exponential and Normal Models

    4 lessons coming soon
    Open the Exponential and Normal Models unit page

    Exponential Waiting Times

    1. Video 64

      exponential distribution, rate, scale parameter, survival function, memorylessness, natural logarithm and exponential quantile

      Builds on The Geometric Distribution, Probability Densities and CDFs, Continuous Expectation and Variance and Uniform Distributions. Also uses derivatives of exponential functions and the product rule.

    Normal Distributions

    1. Video 65

      normal distribution, normal density, standard normal distribution, standardization, 68-95-99.7 rule and normal tail probability

      Builds on Probability Densities and CDFs and Continuous Expectation and Variance. Also uses using a standard normal table or calculator.

    2. Video 66

      z-score, standard normal CDF, symmetry of the standard normal CDF, upper-tail probability, normal quantile, inverse CDF and central probability interval

      Builds on The Normal Distribution.

    Choosing a Continuous Model

    1. Video 67

      distribution family, parameter convention, support of a model, tail behavior and model adequacy

      Builds on Uniform Distributions, Exponential Distributions, The Normal Distribution and Normal Probabilities and Quantiles.

  11. Transformations, Approximations and Monte Carlo

    6 lessons coming soon
    Open the Transformations, Approximations and Monte Carlo unit page

    Changing Variables and Sampling

    1. Video 68

      CDF method, monotone transformation, many-to-one transformation, unbounded density, inverse-CDF sampling and generalized quantile

      Builds on Probability Densities and CDFs, Uniform Distributions, Exponential Distributions and Cumulative Distribution Functions.

    Limit Theorems for Averages

    1. Video 69

      weak law of large numbers for continuous variables, running average, Chebyshev bound for a sample mean, conservative bound and perfectly dependent observations

      Builds on Law of Large Numbers for Discrete Variables, Continuous Expectation and Variance and Mean and Variance of the Sample Mean.

    2. Video 70

      central limit theorem, standardized sum, convergence in distribution, normal approximation for an average, finite positive variance and heavy tails

      Builds on Exponential Distributions, The Normal Distribution, Mean and Variance of the Sample Mean and The Law of Large Numbers for Continuous Variables.

    Exact, Approximate and Simulated Answers

    1. Video 71

      rare-event regime, normal approximation to the binomial, continuity correction, absolute error, relative error and tail accuracy

      Builds on The Binomial Distribution, Poisson Distributions, The Normal Distribution and The Central Limit Theorem.

    2. Video 72

      Monte Carlo method, Monte Carlo estimator, Monte Carlo standard error, probability as an expected indicator, square-root cost of precision and approximate interval for a Monte Carlo estimate

      Builds on Probability Simulation, Transformations and Inverse-CDF Sampling, Simulation Error and Number of Trials and The Central Limit Theorem.

    3. Video 73

      exact model probability, approximation error, simulation error, uncertain model inputs and error interpretation

      Builds on Poisson and Normal Approximations and Monte Carlo Probability and Expectation.

  12. Random Processes and Introductory Markov Chains

    9 lessons coming soon
    Open the Random Processes and Introductory Markov Chains unit page

    Bernoulli Processes and Random Walks

    1. Video 74

      random process, time index, Bernoulli process, cumulative count, first success time, disjoint blocks of trials and count and waiting time events

      Builds on The Bernoulli Distribution, The Geometric Distribution and The Binomial Distribution.

    2. Video 75

      simple random walk, increment, position, drift, variance of a random walk, parity restriction and path probability

      Builds on Bernoulli Processes, The Binomial Distribution and Covariance, Correlation and Sums.

    Poisson Processes

    1. Video 76

      counting process, homogeneous Poisson process, rate of arrivals, independent increments, stationary increments and overlapping counts

      Builds on Poisson Distributions, Exponential Distributions and Bernoulli Processes.

    2. Video 77

      first arrival time, exponential waiting time, interarrival time, kth arrival time, count and waiting time connection and memoryless waiting

      Builds on Poisson Process Basics and Exponential Distributions.

    Markov Chains

    1. Video 78

      Markov chain, state, Markov property, time-homogeneous chain, transition matrix, row-stochastic convention, initial distribution and one-step update

      Builds on Conditional Probability, Conditional Independence and Sequential Evidence and Bernoulli Processes. Also uses matrix multiplication.

    2. Video 79

      n-step transition probability, matrix power, row vector of state probabilities, Chapman-Kolmogorov relation, endpoint probability and visiting probability

      Builds on Markov Chains and Transition Matrices.

    3. Video 80

      stationary distribution, balance equations, probability flow, convergence of the state distribution, periodic chain and nonunique stationary distribution

      Builds on Multi-Step Markov Probabilities.

    Hitting Times and Model Checking

    1. Video 81

      hitting time, hitting probability, absorbing state, first-step analysis, boundary condition, expected time to absorption and infinite hitting time

      Builds on Conditional Expectation and the Tower Rule, Markov Chains and Transition Matrices and Stationary Distributions.

    2. Video 82

      state definition, Markov assumption, refined state, empirical transition matrix, time step, checking time homogeneity and model limitation

      Builds on Stationary Distributions and Hitting Times and Absorption.

  13. Integrated Probability Investigations

    2 lessons coming soon
    Open the Integrated Probability Investigations unit page

    Complete Model Investigations

    1. Video 83

      model specification, joint model from conditional rates, binomial count of alerts, conditional expected work, assumption sensitivity and prediction versus simulated output

      Builds on Bayes’ Theorem and Base Rates, The Binomial Distribution, Conditional Expectation and the Tower Rule, Law of Large Numbers for Discrete Variables and Simulation Error and Number of Trials.

    2. Video 84

      combined probability model, independence of submodels, fixed batch versus fixed time window, checkpoint availability, sensitivity analysis and simulation check of a submodel

      Builds on A Discrete Probability Investigation, Uniform Distributions, Exponential Distributions, The Normal Distribution, Monte Carlo Probability and Expectation, Poisson Process Basics, Arrival Counts and Waiting Times, Stationary Distributions, Hitting Times and Absorption and Choosing and Checking Markov Models.