k-means clustering

One-line definition

k-means partitions $n$ data points into $k$ clusters by minimizing the sum of squared distances from each point to its cluster’s centroid:

$$\underset{{\mu }_1,\dots ,{\mu }_k,C}{\min }\sum _{i=1}^n\Vert x_i-{\mu }_{C(i)}{\Vert }^2.$$

Lloyd’s algorithm (1957) alternates: (a) assign each point to its nearest centroid; (b) recompute each centroid as the mean of its assigned points. Iterate to convergence.

Why it matters

k-means is the canonical clustering algorithm: $O(nkd)$ per iteration, easy to implement, and a good first attempt at any unsupervised structure problem. It is used directly (customer segmentation, vector-quantization codebooks) and as a building block (initialization for GMMs, anchor selection for object detection, sub-sampling for retrieval index training).

The algorithm

Initialize $k$ centroids (random points or k-means++ for stability). Then repeat:

1. Assignment: $C(i)=\arg {\min }_j\Vert x_i-{\mu }_j{\Vert }^2$ for each $i$.
2. Update: ${\mu }_j=\frac{1}{|C^{-1}(j)|}{\sum }_{i\in C^{-1}(j)}{x}_i$ for each $j$.

Stop when assignments don’t change (or change below a threshold). Convergence is guaranteed in finite steps but only to a local minimum. K-means is not convex.

k-means++ initialization

Random initialization sometimes converges to bad local minima. k-means++ (Arthur & Vassilvitskii, 2007):

1. Pick ${\mu }_1$ uniformly at random.
2. For $j=2,\dots ,k$: pick ${\mu }_j$ from data with probability proportional to ${\min }_{j^{\prime }<j}\Vert x-{\mu }_{j^{\prime }}{\Vert }^2$. Points far from existing centers are more likely to be picked.

Gives an $O(\log k)$-approximation to the optimum in expectation. Used as default initialization in scikit-learn and most modern implementations.

Choosing $k$

There is no universally right answer. Heuristics:

• Elbow method: plot total within-cluster sum of squares vs $k$; look for the “elbow” where adding more clusters stops paying off.
• Silhouette score: average similarity of each point to its own cluster vs. nearest other cluster; pick $k$ maximizing it.
• Gap statistic: compare to clustering on uniform random data of the same shape.
• Domain knowledge: often the right answer.

For very large data, run k-means with a few values of $k$ in parallel and pick the one with the best silhouette on a subsample.

Assumptions

k-means implicitly assumes:

• Spherical, equal-variance clusters (penalizes non-spherical clusters by splitting them).
• Equal cluster sizes (large clusters are sometimes split, small ones merged).
• Euclidean distance is meaningful in the input space.

When clusters are elongated, of unequal density, or non-convex, k-means fails. Try GMMs (allows ellipsoidal clusters), DBSCAN (density-based, arbitrary shapes), or spectral clustering.

Mini-batch k-means

For huge $n$, full assignment is expensive. Mini-batch k-means (Sculley, 2010): each iteration uses a small random subset to update centroids. Trades accuracy for huge speedup.

When k-means is and isn’t appropriate

Setting	Verdict
Roughly spherical clusters in low-d	Excellent
Vector quantization codebook	Excellent (k = codebook size)
Image segmentation by color	Good (k = number of colors)
Customer segmentation on raw features	Try, but standardize first
Text or sparse data	Use spherical k-means (cosine distance)
Non-convex shapes (moons, rings)	Fails; use DBSCAN or spectral
Large $k$ (thousands)	Use HNSW-accelerated variants

Common pitfalls

• Forgetting to standardize features. k-means uses Euclidean distance; features with larger scale dominate.
• Random initialization without k-means++. Different runs give different clusterings; report consensus or use k-means++.
• Treating cluster IDs as meaningful. Cluster 0 in one run may be cluster 7 in the next; permutation invariance.
• Using k-means on categorical data without thought. Use k-modes, k-prototypes, or one-hot + k-means with care.
• Reporting one clustering as “the answer.” Run multiple times, take the best by within-cluster SS.