Ward's method means calculating the incremental sum of squares. Half square Euclidean distance is the only distance measure that can be used with this clustering method. Therefore the distance measure is automatically set to Half square Euclidean distance when Ward's method is selected.
Assume that there are three clusters called C1, C2 and C3 including n1, n2 and n3 number of rows (or columns). Clusters C2 and C3 are aggregated to form a new single cluster called C4.
The distance between cluster C1 and the new cluster C4 in the example above is calculated as:
where
See also: