crankshaft/doc/11_kmeans.md

K-Means Functions

k-means clustering is a popular technique for finding clusters in data by minimizing the intra-cluster 'distance' and maximizing the inter-cluster 'distance'. The distance is defined in the parameter space of the variables entered.

CDB_KMeans(subquery text, no_clusters integer)

This function attempts to find no_clusters clusters within the input data based on the geographic distribution. It will return a table with ids and the cluster classification of each point input assuming the_geom is not null-valued. If the_geom is null-valued, the point will not be considered in the analysis.

Arguments

Name	Type	Description
subquery	TEXT	SQL query that exposes the data to be analyzed (e.g., SELECT * FROM interesting_table). This query must have the geometry column name the_geom and id column name cartodb_id unless otherwise specified in the input arguments
no_clusters	INTEGER	The number of clusters to find

Returns

A table with the following columns.

Column Name	Type	Description
cartodb_id	INTEGER	The row id of the row from the input table
cluster_no	INTEGER	The cluster that this point belongs to

Example Usage

SELECT
    customers.*,
    km.cluster_no
FROM
    cdb_crankshaft.CDB_KMeans('SELECT * from customers' , 6) As km,
    customers
WHERE
    customers.cartodb_id = km.cartodb_id

CDB_WeightedMean(subquery text, weight_column text, category_column text)

Function that computes the weighted centroid of a number of clusters by some weight column.

Arguments

Name	Type	Description
subquery	TEXT	SQL query that exposes the data to be analyzed (e.g., SELECT * FROM interesting_table). This query must have the geometry column and the columns specified as the weight and category columns
weight_column	TEXT	The name of the column to use as a weight
category_column	TEXT	The name of the column to use as a category

Returns

A table with the following columns.

Column Name	Type	Description
the_geom	GEOMETRY	A point for the weighted cluster center
class	INTEGER	The cluster class

Example Usage

SELECT
    ST_Transform(km.the_geom, 3857) As the_geom_webmercator,
    km.class
FROM
    cdb_crankshaft.CDB_WeightedMean(
        'SELECT *, customer_value FROM customers',
        'customer_value',
        'cluster_no') As km

CDB_KMeansNonspatial(subquery text, colnames text[], no_clusters int)

K-means clustering classifies the rows of your dataset into no_clusters by finding the centers (means) of the variables in colnames and classifying each row by it's proximity to the nearest center. This method partitions space into distinct Voronoi cells.

As a standard machine learning method, k-means clustering is an unsupervised learning technique that finds the natural clustering of values. For instance, it is useful for finding subgroups in census data leading to demographic segmentation.

Arguments

Name	Type	Description
query	TEXT	SQL query to expose the data to be used in the analysis (e.g., SELECT * FROM iris_data). It should contain at least the columns specified in colnames and the id_colname.
colnames	TEXT[]	Array of columns to be used in the analysis (e.g., Array['petal_width', 'sepal_length', 'petal_length']).
no_clusters	INTEGER	Number of clusters for the classification of the data
id_col (optional)	TEXT	The id column (default: 'cartodb_id') for identifying rows
standarize (optional)	BOOLEAN	Setting this to true (default) standardizes the data to have a mean at zero and a standard deviation of 1

Returns

A table with the following columns.

Column	Type	Description
cluster_label	TEXT	Label that a cluster belongs to, number from 0 to no_clusters - 1.
cluster_center	JSON	Center of the cluster that a row belongs to. The keys of the JSON object are the colnames, with values that are the center of the respective cluster
silhouettes	NUMERIC	Silhouette score of the cluster label
inertia	NUMERIC	Sum of squared distances of samples to their closest cluster center
rowid	BIGINT	id of the original row for associating back with the original data

Example Usage

SELECT
    customers.*,
    km.cluster_label,
    km.cluster_center,
    km.silhouettes
FROM
    cdb_crankshaft.CDB_KMeansNonspatial(
        'SELECT * FROM customers',
        Array['customer_value', 'avg_amt_spent', 'home_median_income'],
        7) As km,
    customers
WHERE
    customers.cartodb_id = km.rowid

Resources

• Read more in scikit-learn's documentation
• K-means basics