# Efficient way to compute distances between centroids from distance matrix

Let us have square symmetric matrix of squared euclidean distances $\bf D$ between $n$ points and vector lengthed $n$ indicating cluster or group membership ($k$ clusters) of the points; a cluster may consist of $\ge1$ point.

What is the most efficient or really efficient (in terms of speed) way to compute distances between the cluster centroids here?

So far I always did Principal Coordinate analysis in this situation. PCoA, or Torgerson's MDS amounts to first converting $\bf D$ into the matrix of scalar products $\bf S$ ("double centering") and then performing PCA of it. This way we create coordinates for the $n$ points in the euclidean space they span. After that, it is easy to compute distances between the centroids the usual way - as you would do it with grouped points x variables data. PCoA has to do eigen-decomposition or SVD of the n x n symmetric positive semidefinite $\bf S$, but $n$ can be quite big. In addition, the task is not a dimensionality reduction one and we don't actually need those orthogonal principal axes. So I have a feeling that these decompositions might be an overkill.

So, do you have knowledge or ideas about a potentially faster way?

Let the points be indexed $x_1, x_2, \ldots, x_n$, all of them in $\mathbb{R}^d$. Let $\mathcal{I}$ be the indexes for one cluster and $\mathcal{J}$ the indexes for another cluster. The centroids are

$$c_\mathcal{I} = \frac{1}{|\mathcal{I}|} \sum_{i\in\mathcal{I}} x_i,\ c_\mathcal{J} = \frac{1}{|\mathcal{J}|} \sum_{j\in\mathcal{J}} x_j$$

and it is desired to find their squared distance $||c_\mathcal{I} - c_\mathcal{J}||^2$ in terms of the squared distances $D_{ij} = ||x_i - x_j||^2$.

Exactly as we would break down sums of squares in ANOVA calculations, an algebraic identity is

$$||c_\mathcal{I} - c_\mathcal{J}||^2 = \frac{1}{|\mathcal{I}||\mathcal{J}|} \left(SS(\mathcal{I \cup J}) -\left(|\mathcal{I}|+|\mathcal{J}|\right) \left(\frac{1}{|\mathcal{I}|}SS(\mathcal{I}) + \frac{1}{|\mathcal{J}|}SS(\mathcal{J})\right)\right)$$

where "$SS$" refers to the sum of squares of distances between each point in a set and their centroid. The polarization identity re-expresses this in terms of squared distances between all points:

$$SS(\mathcal{K}) = \frac{1}{2}\sum_{i,j\,\in\,\mathcal{K}} ||x_i - x_j||^2 = \sum_{i\lt j\,\in\,\mathcal{K}} D_{ij}.$$

The computational effort therefore is $O((|\mathcal{I}|+|\mathcal{J}|)^2)$, with a very small implicit constant. When the clusters are approximately the same size and there are $k$ of them, this is $O(n^2/k^2)$, which is directly proportional to the number of entries in $D$: that would be the best one could hope for.

R code to illustrate and test these calculations follows.

ss <- function(x) {
n <- dim(x)
i <- rep(1:n, n)
j <- as.vector(t(matrix(i,n)))
d <- matrix(c(1,1) %*% (x[,i] - x[,j])^2 , n) # The distance matrix entries for x
sum(d[lower.tri(d)])
}
centroid <- function(x) rowMeans(x)
distance2 <- function(x,y) sum((x-y)^2)
#
# Generate two clusters randomly.
#
n.x <- 3; n.y <- 2
x <- matrix(rnorm(2*n.x), 2)
y <- matrix(rnorm(2*n.y), 2)
#
# Compare two formulae.
#
cat("Squared distance between centroids =",
distance2(centroid(x), centroid(y)),
"Equivalent value =",
(ss(cbind(x,y)) - (n.x + n.y) * (ss(x)/n.x + ss(y)/n.y)) / (n.x*n.y),
"\n")

• Perfect! I must confess that dispite I knew the parallelogram identities I could not myself see clearly the link to my task and to deduce the formula. So many thanks to you. I've already programmed the function (in SPSS) based on your formula for any number of centroids and it is indeed faster with large matrix D than the indirect way via PCoA. – ttnphns May 1 '15 at 8:52
• I'd also add that the formula remains valid if the groups/clusters intersect by the compositions of objects. – ttnphns May 1 '15 at 10:54
• Yes, that is correct: the identity I use does not assume the clusters are disjoint. – whuber May 1 '15 at 14:17
• Just adding a late link: your method in matrix notation, on which I based that function I said above. stats.stackexchange.com/a/237811/3277 – ttnphns Sep 13 '17 at 15:50
• @amoeba $\mathcal K$ refers to any subset of $\{1,2,\ldots, n\}.$ – whuber Jul 27 '18 at 13:15