Distance (\(d_{i,j}\)) between two objects (\(i\) and \(j\)) calculated as the square root of the sum of the squared differences between \(k\) variables (\(x\)) that describe the objects. \[d_{i,j} = \sqrt {\sum\limits_{k\,=\,1}^{k\,=\,N} (x_{i,k} - x_{j,k})^2}\]