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The average silhouette of the data is another useful criterion for assessing the natural number of clusters. The silhouette of a data instance is a measure of how closely it is matched to data within its cluster and how loosely it is matched to data of the neighboring cluster, i.e., the cluster whose average distance from the datum is lowest. [8]
The term "k-means" was first used by James MacQueen in 1967, [2] though the idea goes back to Hugo Steinhaus in 1956. [3]The standard algorithm was first proposed by Stuart Lloyd of Bell Labs in 1957 as a technique for pulse-code modulation, although it was not published as a journal article until 1982. [4]
Directional statistics (also circular statistics or spherical statistics) is the subdiscipline of statistics that deals with directions (unit vectors in Euclidean space, R n), axes (lines through the origin in R n) or rotations in R n. More generally, directional statistics deals with observations on compact Riemannian manifolds including the ...
Similar to other clustering evaluation metrics such as Silhouette score, the CH index can be used to find the optimal number of clusters k in algorithms like k-means, where the value of k is not known a priori. This can be done by following these steps: Perform clustering for different values of k. Compute the CH index for each clustering result.
In data mining, k-means++ [1] [2] is an algorithm for choosing the initial values (or "seeds") for the k-means clustering algorithm. It was proposed in 2007 by David Arthur and Sergei Vassilvitskii, as an approximation algorithm for the NP-hard k-means problem—a way of avoiding the sometimes poor clusterings found by the standard k-means algorithm.
Normal curve equivalent; Normal distribution; Normal probability plot – see also rankit; Normal score – see also rankit and Z score; Normal variance-mean mixture; Normal-exponential-gamma distribution; Normal-gamma distribution; Normal-inverse Gaussian distribution; Normal-scaled inverse gamma distribution; Normality test; Normalization ...
About 68% of values drawn from a normal distribution are within one standard deviation σ from the mean; about 95% of the values lie within two standard deviations; and about 99.7% are within three standard deviations. [8] This fact is known as the 68–95–99.7 (empirical) rule, or the 3-sigma rule.
For medium size samples (<), the parameters of the asymptotic distribution of the kurtosis statistic are modified [37] For small sample tests (<) empirical critical values are used. Tables of critical values for both statistics are given by Rencher [38] for k = 2, 3, 4.