**Sum of squared values:** $\sum x_i^2$ means square each value, then add them up. - **Sum of squared differences from the mean:** $\sum (x_i - \bar{x})^2$. This is the numerator in the variance formula. It measures how spread out the data is. - **Double summation:** $\sum_{i=1}^{m} \sum_{j=1}^{n} a