Relationships Between Variables

Measuring and modelling how two quantities vary together.

In suggested reading order:

1. Covariance. A measure of whether two variables tend to be above and below their means at the same time.
2. Correlation. A unit-free number between −1 and 1 that measures the strength and direction of a linear relationship between two variables.
3. Simple Linear Regression. Fitting a straight line to predict one variable from another by minimizing the sum of squared residuals.