Correlation

Is it between -1 and 1
Formula \(r = \frac{\sum_{i=1}^{n} (x_i - \overline{x})(y_i - \overline{y})}{\sqrt{\left(\frac{\sum_{i=1}^{n} (x_i - \overline{x})^2}{n-1}\right) \left(\frac{\sum_{i=1}^{n} (y_i - \overline{y})^2}{n-1}\right)}}\)
The formula has the standard deviation (Dispersion) for y and x and the Degrees of Freedom to make it unbiased
On the correlation matrix, the diagonal == 1
We can use corrplot/heatmaps
Correlation is sensitive to outliers

Spearman/Kendalls

They are based on ranks e are robust to outliers
In addition, they can deal with some non-linearity as well
It is suitable for small datasets