Khan Academy - Variance
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Khan Academy's free video lessons and practice exercises on measuring spread, covering population and sample variance, standard deviation, and mean absolute deviation, so learners can compute these measures by hand and interpret what they say about a dataset's variability.
More resources on Variance & Covariance
probabilitycourse.com
ProbabilityCourse.com is an online learning resource that provides a structured probability course with clear explanations, worked examples, and practice problems covering topics from basics to advanced theory.
Wolfram MathWorld
MathWorld is an online mathematics encyclopedia from Wolfram Research offering detailed, browsable articles on topics across the math spectrum, including algebra, geometry, calculus, and number theory. Each entry includes definitions, theorems, formulas, diagrams, worked examples, and links to further reading.
Seeing Theory
An interactive, D3.js-based walkthrough of probability and statistics in six chapters, from basic probability and distributions through frequentist inference, Bayesian inference and regression analysis. Parameters are manipulable, so sampling behaviour and fit are visible directly.
Stanford EE278 Lecture Notes 3: Random Vectors, Mean and Covariance Matrix
Graduate lecture notes generalizing variance to the covariance matrix of a random vector, proving symmetry and positive semidefiniteness, then deriving coloring and whitening transforms and the Gaussian random vector.
Covariance, Clearly Explained
Visual walkthrough starting from variance, then building covariance from paired scatter data, showing positive, negative and near-zero cases and explaining why the raw covariance value resists interpretation without correlation.
Statistics 110 Lecture 21: Covariance and Correlation
Harvard lecture introducing covariance and correlation, then using bilinearity to compute the variance of a sum of dependent variables, worked through to the variance of a Hypergeometric random variable.