> [!definition] > > A correlation is a synchronised pattern of variation between two [[Random Variable|random variables]] $X$ and $Y$, from which a [[Scientific Model|model]] can be built to (roughly) predict one variable based on the other without [[Science/Scientific Method/Measurement|measurement]]. > [!definition] > > Let $X$ and $Y$ be [[Random Variable|random variables]]. Then their **correlation** is the [[Expectation|expectation]] of their product $\ev(XY)$.