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What is K-means Clustering in Machine Learning?

  • Neelam Tyagi
  • Mar 14, 2020
  • Updated on: Feb 09, 2021
What is K-means Clustering in Machine Learning? title banner

Clustering is the immense pool of technologies to catch classes of observations (known as clusters) under a dataset provided, that contribute identical features. 


Clustering is arranged in a way that each observation in the same class possesses similar characteristics and observation of separate groups shows dissimilarity in characteristics. 


As a part of the unsupervised learning method, clustering attempts to identify a relationship between n-observations( data points) without being trained by the response variable.


With the intent of obtaining data points under the same class as identical as possible, and the data points in a separate class as dissimilar as possible.


Basically, in the process of clustering, one can identify which observations are alike and classify them significantly in that manner. Keeping this perspective in mind, k-means clustering is the most straightforward and frequently practised clustering method to categorize a dataset into a bunch of k classes (groups).



Table of Content


  1. Introduction

  2. What is K-means clustering?

  3. Features and Limitations

  4. Expectation-Maximization: K-means Algorithm

  5. Working of K-means clustering

  6. Applications of K-means clustering

  7. K-means vs Hierarchical clustering





Beginning with Unsupervised Learning, a part of machine learning where no response variable is present to provide guidelines in the learning process and data is analyzed by algorithms itself to identify the trends. 


Opposite to that, supervised learning is where existing data is already labelled and you know which behaviour you want to recognize from new datasets, unsupervised learning doesn’t exhibit labelled dataset and algorithms are there to explore relationships and patterns in the data. You can learn more about these types of machine learning here.


It is a known fact that the data and information are usually obscured by noise and redundancy so making it into groups with similar features is the decisive action to bring some insights.


One of the excellent methods in unsupervised machine learning treated for data classification, k-means suits well for exploratory data analysis to understand data perfectly and get inferences from all data types despite the data in the form of images, text content or numeric, k-means works flexibly. 


( Prefered blog: (GAN) in Unsupervised Machine Learning)



What is K-means Clustering?


K-means algorithm explores for a preplanned number of clusters in an unlabelled multidimensional dataset, it concludes this via an easy interpretation of how an optimized cluster can be expressed. 


Primarily the concept would be in two steps;


  • Firstly,  the cluster centre is the arithmetic mean (AM) of all the data points associated with the cluster.
  • Secondly, each point is adjoint to its cluster centre in comparison to other cluster centres. These two interpretations are the foundation of the k-means clustering model.


You can take the centre as a data point that outlines the means of the cluster, also it might not possibly be a member of the dataset. 


In simple terms, k-means clustering enables us to cluster the data into several groups by detecting the distinct categories of groups in the unlabelled datasets by itself, even without the necessity of training of data.


This is the centroid-based algorithm such that each cluster is connected to a centroid while following the objective to minimize the sum of distances between the data points and their corresponding clusters. 


As an input, the algorithm consumes an unlabelled dataset, splits the complete dataset into k-number of clusters, and iterates the process to meet the right clusters, and the value of k should be predetermined. 


Specifically performing two tasks, the k-means algorithm

  • Calculates the correct value of K-centre points or centroids by an iterative method

  • Assigns every data point to its nearest k-centre, and the data points, closer to a particular k-centre, make a cluster. Therefore, data points, in each cluster, have some similarities and far apart from other clusters. 


You can learn k-means clustering by the example given in the following video,

Key Features of K-means Clustering


Find below some key features of k-means clustering;


  1. It is very smooth in terms of interpretation and resolution.

  2. For a large number of variables present in the dataset, K-means operates quicker than Hierarchical clustering. 

  3. While redetermining the cluster centre, an instance can modify the cluster. 

  4. K-means reforms compact clusters.

  5. It can work on unlabeled numerical data.

  6. Moreover, it is fast, robust and uncomplicated to understand and yields the best outcomes when datasets are well distinctive (thoroughly separated) from each other.


Limitations of K-means Clustering


The following are a few limitations with K-Means clustering;


  1. Sometimes, it is quite tough to forecast the number of clusters, or the value of k.

  2. The output is highly influenced by original input, for example, the number of clusters.

  3. An array of data substantially hits the concluding outcomes.

  4. In some cases, clusters show complex spatial views, then executing clustering is not a good choice.

  5. Also, rescaling is sometimes conscious, it can’t be done by normalization or standardization of data points, the output gets changed entirely.


(Recommended blog: Machine Learning tools)


Disadvantages of K-means Clustering


  1. The algorithm demands for the inferred specification of the number of cluster/ centres.

  2. An algorithm goes down for non-linear sets of data and unable to deal with noisy data and outliers.

  3. It is not directly applicable to categorical data since only operatable when mean is provided.

  4. Also, Euclidean distance can weight unequally the underlying factors.

  5. The algorithm is not variant to non-linear transformation, i.e provides different results with different portrayals of data.



Expectation-Maximization: K-means Algorithm


K-Means is just the Expectation-Maximization (EM) algorithm, It is a persuasive algorithm that exhibits a variety of context in data science, the E-M approach incorporates two parts in its procedure;


  1. To assume some cluster centres,
  2. Re-run as far as transformed;
  • E-Step: To appoint data points to the closest cluster centre,

  • M-Step: To introduce the cluster centres to the mean.


Where the E-step is the Expectation step, it comprises upgrading forecasts of associating the data point with the respective cluster. 


And, M-step is the Maximization step, it includes maximizing some features that specify the region of the cluster centres, for this maximization, is expressed by considering the mean of the data points of each cluster. 


In account with some critical possibilities, each reiteration of E-step and M-step algorithm will always yield in terms of improved estimation of clusters’ characteristics. 


K-means utilize an iterative procedure to yield its final clustering based on the number of predefined clusters, as per need according to the dataset and represented by the variable K. 


For instance, if K is set to 3 (k3), then the dataset would be categorized in 3 clusters if k is equal to 4, then the number of clusters will be 4 and so on. 


The fundamental aim is to define k centres, one for each cluster, these centres must be located in a sharp manner because of the various allocation causes different outcomes. So, it would be best to put them as far away as possible from each other.


Also, The maximum number of plausible clusters will be the same as the total number of observations/features present in the dataset.



Working of K-means Algorithm


Don’t you get excited !!! Yes, you must be, let’s move ahead with the notion of working algorithm.


By specifying the value of k, you are informing the algorithm of how many means or centres you are looking for. Again repeating, if k is equal to 3, the algorithm accounts it for 3 clusters. 


Following are the steps for working of the k-means algorithm;


  • K-centres are modelled randomly in accordance with the present value of K.
  • K-means assigns each data point in the dataset to the adjacent centre and attempts to curtail Euclidean distance between data points. Data points are assumed to be present in the peculiar cluster as if it is nearby to centre to that cluster than any other cluster centre. 
  • After that, k-means determines the centre by accounting the mean of all data points referred to that cluster centre. It reduces the complete variance of the intra-clusters with respect to the prior step. Here, the “means” defines the average of data points and identifies a new centre in the method of k-means clustering.

The image interprets the clustering of some objects on the basis of their shapes, they can also be classified on the basis of color.

Clustering of data points (objects in this case)

  • The algorithm gets repeated among the steps 2 and 3 till some paradigm will be achieved such as the sum of distances in between data points and their respective centres are diminished, an appropriate number of iterations is attained, no variation in the value of cluster centre or no change in the cluster due to data points. 


Stopping Criteria for K-Means Clustering


On a core note, three criteria are considered to stop the k-means clustering algorithm


  1. If the centroids of the newly built clusters are not changing


An algorithm can be brought to an end if the centroids of the newly constructed clusters are not altering. Even after multiple iterations, if the obtained centroids are same for all the clusters, it can be concluded that the algorithm is not learning any new pattern and gives a sign to stop its execution/training to a dataset. 


  1. If data points remain in the same cluster


The training process can also be halt if the data points stay in the same cluster even after the training the algorithm for multiple iterations.


  1. If the maximum number of iterations have achieved


At last, the training on a dataset can also be stopped if the maximum number of iterations is attained, for example, assume the number of iterations has set as 200, then the process will be repeated for 200 times (200 iterations) before coming to end. 




Applications of K-means Clustering


The concern of the fact is that the data is always complicated, mismanaged, and noisy. The conditions in the real world cast hardly the clear picture to which these types of algorithms can be applied. Let’s learn where we can implement k-means clustering among various 


  1. K-means clustering is applied in the Call Detail Record (CDR) Analysis. It gives in-depth vision about customer requirements and satisfaction on the basis of call-traffic during the time of the day and demographic of a particular location.

  1. It is used in the clustering of documents to identify the compatible documents in the same place. 

  2. It is deployed to classify the sounds on the basis of their identical patterns and segregate malformation in them.   

  1. It serves as the model of lossy images compression technique, in the confinement of images, K-means makes clusters pixels of an image in order to decrease the total size of it. 

  1. It is helpful in the business sector for recognizing the portions of purchases made by customers, also to cluster movements on apps and websites.

  1. In the field of insurance and fraud detection on the basis of prior data, it is plausible to cluster fraudulent consumers to demand based on their proximity to clusters as the patterns indicate. 



K-means vs Hierarchical Clustering 


  1. K-means clustering produces a specific number of clusters for the disarranged and flat dataset, where Hierarchical clustering builds a hierarchy of clusters, not for just a partition of objects under various clustering methods and applications.

  1. K-means can be used for categorical data and first converted into numeric by assigning rank, where Hierarchical clustering was selected for categorical data but due to its complexity, a new technique is considered to assign rank value to categorical features.

  1. K-means are highly sensitive to noise in the dataset and perform well than Hierarchical clustering where it is less sensitive to noise in a dataset.

  1. Performance of the K-Means algorithm increases as the RMSE decreases and the RMSE decreases as the number of clusters increases so the time of execution increases, in contrast to this, the performance of Hierarchical clustering is less. 

  1. K-means are good for a large dataset and Hierarchical clustering is good for small datasets.





K-means clustering is the unsupervised machine learning algorithm that is part of a much deep pool of data techniques and operations in the realm of Data Science. It is the fastest and most efficient algorithm to categorize data points into groups even when very little information is available about data. 


More on, similar to other unsupervised learning, it is necessary to understand the data before adopting which technique fits well on a given dataset to solve problems. Considering the correct algorithm, in return, can save time and efforts and assist in obtaining more accurate results. 

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