Mahalanobis Distance
Approximates fully blocked experiment
Distance \((X_c,X_t)\) = \(\sqrt{(X_c - X_t)'S^{-1}(X_c - X_t)}\)
where \(S^{-1}\) standardize the distance
In application we use Euclidean distance.
Prune unused control units, and prune matches if distance > caliper