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TensorFlow or Theano?

February 20, 2021 / / 1 Comment

TensorFlow or Theano – TensorFlow, along with PyTorch, is currently the best known and most widely used machine learning framework. However, the choice of tool should never depend on one’s own preferences, but should be adapted to the data to be examined. Especially in the Big data area, this can prevent a decisive loss of performance. It is therefore also worthwhile to look off the beaten track and to look at other frameworks and libraries in addition to the top dogs.
Theano is one such open source Python library. In the following article, we will introduce both tools and explain the differences.

What is Tensorflow?

The open source framework TensorFlow is the direct successor of Google’s first deep learning tool DistBelief and primarily also forms the basis for neural networks in the environment of language and image processing tasks. With TensorFlow, own models can be developed and processed, but also pre-trained models can be accessed. TF runs on a variety of platforms and is implemented in Python and C++.

TensorFlow vs Theano - This figure shows the hierarchy of the TensorFlow framework.
Hierarchy of TensorFlow toolkits

TF offers low-level APIs for CPU, GPU or TPU. In this way, the hardware resources can be optimally adapted to the process through dynamic allocations.
In addition to the low level APIs, there are also various high level APIs, such as Keras, one of the best known and most frequently used. If you want to know more about Keras, check out our article on the topic.

Framework Architecture

Mainly, the TensorFlow framework can be divided into the components needed for training, where the models are prepared for field use, and for the final deployment, for example on mobile and IoT devices with TensorFlowLite. To simplify the training, TensorFlow offers the developer some useful services besides the already mentioned dynamic allocation. For example, a premade estimator offers a high-level representation of a complete model.Via the TensorFlow Hub, a kind of repository, even trained machine learning models can be other language bindings can be accessed.

TensorFlow vs Theano - This figure shows the structure of the TensorFlow framework.
TensorFlow or Theano – Structure of the TensorFlow Framework

The TensorBoard and StoredModels services act as connecting elements between training and deployment. TensorBoard is the visualization toolkit of TensorFlow with which the experiment results can be visualized. So here it is more of a monitoring solution for the human interface. With the StoredModels both deployment services and training services can share the models. This service thus forms a kind of intermediary, but contains a complete TensorFlow program, including all weights and calculations.

TensorFlow – Data Structure

Neural networks are represented by directed cycle-free graphs. These graphs can be represented and computed beyond the computer limits of training. A graph basically consists of nodes connected by edges. The extent to which the nodes are interconnected also usually determines the learning procedure and thus the structure of an artificial neural network.
The inputs and outputs of the individual calculation steps represent multidimensional data arrays, so-called tensors.

This figure shows the basic tensor structure
Tensor Principle

The mathematical term tensor corresponds to a generalization of vectors and matrices. It is thus an elementary data structure for data representation and processing. In TensorFlow the implementation is done as multidimensional arrays . A vector thus corresponds to a one-dimensional tensor.
Additional dimensions can be added to a tensor up to infinity. Common tensor types are 3-dimensional tensors for time series, images are usually 4-dimensional, and videos are 5-dimensional tensors.

pytorch training 2
Tensors and neural networks

TensorFlow methods manipulate tensors for linear algebra operations. These processes can be executed with high performance by moving the tensor objects to the graphics card memory or tensor optimized TPUs.

TensorFlow – Training

The training itself then proceeds in such a way that training data are iteratively fed into the computers and at the same time the weights within the graph are varied. The output is then approximated to a target output value. To this end, separate test data can be used to periodically verify that the training is effective for arbitrary or different input data.

The figure shows the sequence of the training of a neural network
Training procedure

Theano – Old but Gold

Theano is an open source Python library for machine learning and neural network programming, and compiler for mathematical expression computation. It was released back in 2007 by the Montreal Institute for Learning Algorithms (MILA) at the University of Montreal.
It is particularly suitable for the definition, optimization and evaluation of mathematical expressions involving multidimensional arrays. For this purpose, Theano accesses the NumPy program library for dealing with matrices, large multidimensional arrays and vectors. First, read our article on NumPy. Here we introduce you to this elementary Python library and explain its basic data management.


Mathematical expressions are programmed and symbolized in Theano using a NumPy-like syntax.
The calculation instructions are done in C++ or CUDA code, thus very close to the machine and accordingly very efficient on CPUs or graphics processing units (GPUs).
Theano can also be used, like TensorFlow as a backend for the framework Keras. Keras thus forms an intersection for both technologies.

Graph Structure

Unlike TensorFlow, Theano focuses on supporting symbolic matrix expressions rather than tensors as a basic data type. Although all kinds of Python objects are supported, basic tensor functionality can be used with Theano, but these operations are not as optimized as with TensorFlow.

Theano performs the symbolic mathematical calculations are executed as graphs. These graphs are composed of interconnected Apply, Variable and Op nodes.

TensorFlow vs Theano - Overview structure of a Theano graph
TensorFlow or Theano – Overview structure of a Theano graph

The Op node represents a particular computation on a particular type of input that produces a particular type of output. It thus corresponds to the definition of a computation.


The centrally located Apply node represents the application of an Op to some variables, that is, the application of computations to the current data, and is used to represent a computation graph. Each op is responsible for knowing how to build an Apply node from a list of inputs and thus determines the determines the function and transformation.
An Apply node additionally consists of the input or output fields. The inputs represent the arguments of the function, and the outputs represent the return values of the function.

The Apply nodes then refer to their input and output variables, the main data structure, in the graph via their input and output fields, respectively.
These Variable Nodes are defined by various fields. The variable type, the owner, which can be None or an Apply node of which the variable is an output, the index and the variable name.

TensorFlow or Theano?

All in all, both technologies have their advantages and disadvantages. But both have their raison d’être. Here, too, the data set provides the tools.

In the table below, we have listed all the important points of difference in detail.

TensorFlow vs Theano - This table compares both tools in detail.
TensorFlow or Theano – Comparision

Especially when it comes to tensor processing, as in image processing and sound recognition, TensorFlow with its optimized operations should be the first choice. Another tensor-based alternative to the Google solution is PyTorch from Facebook. In this article we compared these two tools.
Despite its age, Theano is a high-performance and modern alternative for the calculation of matrix expressions.

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PyTorch vs TensorFlow – Facebook vs Google – Duel of the Giants

February 6, 2021 / / 0 Comments

In recent years, the field of data science has been able to access increasingly powerful analysis methods thanks to increasingly high-performance hardware. Google’s Tensorflow has been the benchmark for editing machine learning and modeling deep learning methods. It still has the most freedom today. But a wide range of options often creates a high barrier to entry.

PyTorch vs TensorFlow – With the 2 years younger, also Python-based, open source package PyTorch, Facebook now wants to knock Tensorflow off its throne. It has been steadily gaining popularity for years due to its simplicity and features.
In this article, we will clarify what is in the package and whether it can really compete with Tensorflow.

What is PyTorch?

Pytorch is one of the most popular open source Python packages for scientific computing and neural network development/training.
It was developed by Facebook in 2016 and is based on the Torch library written in Lua. A NumPy-like tensor library that provides rich GPU support to enable accelerated neural network learning. PyTorch is also often referred to as the library of the same name. More about this in the section “Libraries”.
Tensors form the elementary data structures for PyTorch, similar to Tensorflow.

PyTorch vs TensorFlow – Tensors form the basis for both!

The mathematical term tensor corresponds to a generalization of vectors and matrices. It is thus an elementary data structure for data representation and processing. In PyTorch the implementation is done as multidimensional arrays . A vector thus corresponds to a one-dimensional tensor.

PyTorch vs TensorFlow - the figure schematically shows the principle behind tensors.
PyTorch vs TensorFlow – Tensor Principle

More dimensions can be added to a tensor up to infinity. Common types of tensors are 3 dimensional tensors for time series, images are usually 4 dimensional and videos are five dimensional tensors.

PyTorch vs TensorFlow -The figure shows the role of tensors in the training of neural networks in PyTorch.
PyTorch vs TensorFlow – Tensors and neural networks

PyTorch methods manipulate tensors for linear algebra operations. These processes can run at high performance by moving the tensor objects into the graphics card memory.

PyTorch Libraries

Pytorch offers the possibility to include specific libraries. This way the program can be kept lean and only make references to needed code.
The PyTorch library itself is an optimized tensor library for deep learning on both GPUs and CPUs.
By including another library, PyTorch can also compute on TPUs.


Depending on the data type, different libraries can be loaded, which provide optimized methods and pre-modeled prototypes for analysis. Torchaudio offers besides the usual audio transformation methods also data sets for training. With torchtext large language packages can be accessed and with torchvision images can be analyzed.

PyTorch vs TensorFlow -The figure shows all PyTorch Libraries.
PyTorch Libraries

With TorchElastic, training jobs can be managed and elatically distributed, for example, to shared capacities.

PyTorch features

Through accelerated tensor analysis via allocation to GPUs, PyTorch achieves high flexibility and high speed in Deep Learning algorithms. Beyond this, PyTorch offers through its Python base unlimited compatibilities to powerful Python libraries, such as NumPy and SciPy and to the Cython programming language. Here we have collected the most important Python open source data management and analysis libraries.


Reverse-mode auto differentiation allows developers to modify network behavior at will, without delay or overhead. This allows for essential acceleration of research iterations.
The 8-bit quantization model ensures efficient deployment on servers and edge devices, and PyTorch Mobile can be used to develop for Android and iOS environments.
Other features include named tensor, artificial neural network pruning, and parallel training of models with remote procedure call.

PyTorch can access TorchServe, an open source server from Facebook, and is fully compatible with cloud provider Amazon Web Services (AWS). If you don’t know what AWS is, read our article on the subject.

PyTorch offers a hybrid frontend as an additional feature. This offers the possibility to choose between two modes. The Eager and the Graph mode. The eager mode primarily offers usability and flexibility, while the graph mode offers better speed, optimization and functionality in a C++ runtime environment. PyTorch also allows conversion with the Hybrid frontend. This allows models to be developed in eagermode and then transferred to graph mode for production.

PyTorch has unlimited access to ONNX (Open Neural Network Exchange) compatible platforms. ONNX is an open source project jointly developed by Microsoft, Amazon, and Facebook, among others, that enables the exchange of AI models between different tools.

PyTorch vs Tensorflow

Duel of the Giants

Just like the Facebook solution, Tensorflow works with the tensor data type. PyTorch scores with its simplicity and effective memory usage. Tensorflow, on the other hand, is much more scalable and thus better suited for production models. An essential difference was originally that with PyTorch the graph structure is defined during execution, while with Tensorflow it is first defined and then executed. Here, however, Tensorflow has now followed with its own eager mode. However, this is not yet fully developed at this stage.

PyTorch vs TensorFlow - The figure shows the main differences between Google's Tensorflow and Facebook's PyTorch
Tensorflow vs PyTorch

PyTorch vs Tensorflow – Who is ahead now?

It remains an exciting head-to-head race. Despite its recent development history, PyTorch has already made up a lot of ground and is interesting in an entrepreneurial context precisely because of its user-friendliness. As is often the case, however, it is not a question of which solution will come out on top, but rather of the principle that competition stimulates business. In the end, competitive pressure leads to great new innovations and exciting new tools.

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Supervised vs Unsupervised vs Reinforcement Learning – The fundamental differences

January 24, 2021 / / 1 Comment

Supervised vs Unsupervised vs Reinforcement Learning – The three main categories of machine learning. Why these boundaries have been drawn and what they look like will be discussed in this article. The knowledge about this is an elementary part to understand machine learning correctly and to be able to apply it to data in a meaningful way.

This figure contrasts Supervised vs Unsupervised vs Reinforcement Learning.
Supervised vs Unsupervised vs Reinforcement Learning – Overview

Supervised vs Unsupervised vs Reinforcement Learning – Machine Learning Categories

Machine learning is a branch of artificial intelligence. While AI deals with the functioning of artificial intelligence and compares it with the functioning of the human brain, machine learning is a collection of mathematical methods of pattern recognition. If you want to know more about the differences between Machine Learning, AI and Deep Learning, read our article on the subject. IT systems should be given the ability to automatically learn from experience and improve. Algorithms play a central role here. These can be classified into different learning categories.

In the following figures the three main categories of machine learning methods are shown.

This figure shows Supervised vs Unsupervised vs Reinforcement Learning in the machine learning context.
Supervised vs Unsupervised vs Reinforcement Learning – Machine Learning Context

In the meantime, there are many more categories, some of which are hybrids of the individual main categories. One example is semi-supervised learning. This is certainly also a major machine learning topic, but has been left out for the time being for the sake of simplicity.

What is supervised learning?

In supervised learning, the machine learning algorithm iteratively learns the dependencies between data points. The output to be learned is specified in advance and the learning process is supervised by matching the predictions. How the The optimized algorithm is to apply the learned patterns to unknown data to make predictions.

Supervised vs Unsupervised vs Reinforcement Learning - This figure shows the basic principle of supervised learning.
Supervised vs Unsupervised vs Reinforcement Learning – Supervised Learning

Supervised learning methods can be applied to regression, i.e., prediction, or trend prediction, as well as classification problems.

What is supervised classification?

In classification, abstract classes are formed in order to delimit and order data in a meaningful way. For this purpose, objects are obtained on the basis of certain similar characteristics and structured among each other.

Decision trees can be used as prediction models to create a hierarchical structure, or the feature values can be assigned as class labels and in the form of a vector.

In the following figure the most important supervised classification algorithms are listed.

Supervised vs Unsupervised vs Reinforcement Learning - This figure shows the main algorithms of supervised learning.
Supervised vs Unsupervised vs Reinforcement Learning – Main Algorithms of Supervised Learning.

What is supervised regression?

On the other hand, supervised regression algorithms can be used to make predictions and infer causal relationships between independent and dependent variables.
For example, linear regression can be used to fit the data to a straight line or, conversely, to fit a line to the data object.
We have discussed the exact process of linear regression here in this article.

What is unsupervised learning?

In unsupervised learning, patterns are determined in data without initial patterns and relationships being known.
Especially in complex tasks, these methods can be useful to find solutions that would hardly be solvable by hand. An example is autonomous driving, or large biochemical systems with many interactions.
One key to success is a huge data set. The more data available, the more accurate models can be created.

Supervised vs Unsupervised vs Reinforcement Learning - This figure shows the basic principle of unsupervised learning.
Supervised vs Unsupervised vs Reinforcement Learning – Unsupervised Learning

In unsupervised machine learning methods, two basic principles, which also classify the algorithms used, can be distinguished. The clustering and the dimensional reduction.

What is unsupervised clustering?

The main goal of unsupervised clustering is to create collections of data elements that are similar to each other, but dissimilar to elements in other clusters. The figure below shows some of the main clustering algorithms.

Supervised vs Unsupervised vs Reinforcement Learning - This figure shows the main algorithms of unsupervised learning.
Supervised vs Unsupervised vs Reinforcement Learning – Main algorithms of unsupervised learning.

The clustering algorithms differ primarily in the cluster creation process, but also in the definition of such clusters. Thus, the relationships between clusters can also be used and hierarchical relationships can be explored.

What is unsupervised dimensional reduction?

With a high number of features, high dimensional relations can be translated low dimensional with these transformation methods. The goal is to keep the loss of information as small as possible.
The reduction methods can be divided into two main categories: Methods from linear algebra and from manifold learning.

Manifold learning is an approach to nonlinear dimensionality reduction. Algorithms for this task are based on the idea that they can learn the dimensionality of the data without a given classification and project it in a low-dimensional way.
For example, from the field of linear algebra, matrix factorization methods can be used for dimensionality reduction.

What is reinforcement learning?

In reinforcement learning, a program, a so-called agent, should independently develop a strategy to perform actions in an environment. For this purpose, positive or negative reinforcements are conveyed, which describe the interaction interactions of the agent with the environment. In other words, immediate feedback on an executed task. The program should maximize rewards or minimize punishments. The environment is a kind of simulation scenario that the agent has to explore.
The following figure describes the interactions of all components of a reinforcement learning process.

Supervised vs Unsupervised vs Reinforcement Learning - This figure shows the main principle of reinforcement learning.
Supervised vs Unsupervised vs Reinforcement Learning – Main principle of reinforcement learning.

There are two basic types of reinforcement learning.
Namely, whether the environment is model-based or not.
In model-based RL, the agent uses predictions of the environment response during learning or action.
If no model is available, the data is generated by trial and error.

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t-SNE – Great Machine Learning Algorithm for Visualization of High-Dimensional Datasets

January 14, 2021 / / 0 Comments

The machine learning algorithm t-Distributed Stochastic Neighborhood Embedding, also abbreviated as t-SNE, can be used to visualize high-dimensional datasets. Each high-dimensional information of a
data point is reduced to a low-dimensional representation. However, the information about existing neighborhoods should be preserved.

So this technique is another tool you can use to create meaningful groups in unordered data collections based on the unifying data properties. If you don’t know what cluster algorithms are, check out this article. Here we present 5 machine learning methods that you should know.
As shown in the following figure, the data should be represented grouped in 2-dimensional space.

The figure shows the data clusters generated by t-Distributed Stochastic Neighborhood Embedding (T-SNE) in 2-dimensional space.
Data clusters generated by t-Distributed Stochastic Neighborhood Embedding (T-SNE)

But how does the algorithm work and what are its strengths? In order to understand its function, we need to look at the origin of the technology.

What is the Stochastic Neighbor Embedding (SNE) Algorithm?

The basis of the t-Distributed Stochastic Neighborhood Embedding algorithm is originally the Stochastic Neighbor Embedding (SNE) algorithm. This converts high-dimensional Euclidean distances into similarity probabilities between individual data points.
The probability with which an object occurs next to a potential neighbor must be calculated.
The dissimilarities between two high-dimensional data points can be explained with a distance matrix, corresponding to the squared Euclidean distance.
A conditional probability is calculated for the low-dimensional correspondence.
This determines the similarity of the two data points on the low-dimensional map.

In order to achieve the closest possible correspondence between the two distributions pij and
qij, a Kullback-Leibler divergence (KL) over all neighbors of each data point is computed as a cost function C. Large costs are incurred for distant data points.

t-Distributed Stochastic Neighborhood Embedding: minimized cost function: sum of the Kullback-Leibler divergences between the original and the induced distribution over the neighbors of an object.
Minimized Cost function: sum of the Kullback-Leibler divergences between the original and the induced distribution over the neighbors of an object.

A gradient descent method is used to optimize the cost function. However, this optimization method converges very slowly. In addition, a so-called crowding problem arises.

If a high dimensional data set is linearly approximated in a small scale, then it cannot be reduced to a lower dimension with a local scaling algo-
rithm to a lower dimension.

What makes the t-Distributed Stochastic Neighborhood Embedding (t-SNE) Algorithmt work?

The t-Distributed Stochastic Neighbor
Embedding (t-SNE) algorithm starts here. On the one hand, a simplified symmetric cost function is used.

The figure shows the simplified symmetric cost function used in t-Distributed Stochastic Neighborhood Embedding.
t-SNE: simplified symmetric cost function

Here, only one KL is minimized over a common probability distribution of all
high, and low dimensional data is minimized.

On the other hand, the similarity of the low-dimensional data points is computed with a Student’s t-distribution and a degree of freedom of one. This can be optimized quickly and is stable to the crowding problem.
stable against the crowding problem.

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AI vs Machine Learning vs Deep Learning – It’s almost harder to understand all the acronyms around AI than the technology itself.

January 5, 2021 / / 5 Comments

It’s almost harder to understand all the acronyms around Artificial Intelligence (AI) than the technology itself.
AI vs Machine Learning vs Deep Learning – These terms are often carelessly mixed together. But what are actually the differences? In this article, we will introduce you to all Three fields, because even though there is overlap, they differ.
It should be important for you to know these differences, as each discipline describes different stages of a data analysis pipeline.

AI vs Machine Learning vs Deep Learning

In the following figure, we have schematically shown you the individual fields in their context. As you can see, the individual disciplines surround each other and form an onion-like layered model.

Schematic representation of ai vs Machine Learning vs Deep Learning.
AI vs Machine Learning vs Deep Learning – Contextual representation of the AI disciplines

The figure clearly shows that there are relationships between individual disciplines. AI is to be understood as a generic term and thus includes the other fields. The deeper you go in the model, the more specific the tasks become. In the following, we will follow this representation and work our way from the outside to the inside.

Artificial intelligence

All disciplines are encompassed by the term AI. It is a science that explores ways to build intelligent programs and machines that can perceive, reason, act, and solve problems creatively. To this end, it attempts to model how the human brain works.
The following figure shows that AI can basically be divided into two categories.

AI vs Machine Learning vs Deep Learning Ability and functionally based AI types simply explained
Types of AI

Classification is about measuring the performance of AI based on how well it is able to replicate the human-like brain. In the Based on Functionality category, AI is classified based on how well it matches the human way of thinking. In the second category, it is evaluated based on human intelligence. Within these categories, there are still some subgroups that correspond to an index.

AI vs Machine Learning

So what is the first subcategory Machine Learning and how does it differ from AI?
While AI deals with the functioning of artificial intelligence and compares them with the functioning of the human brain, machine learning is a collection of mathematical methods of pattern recognition. It is about how a system is given the ability to automatically learn and improve from experience. Various algorithms (e.g., neural networks) are used for this purpose. In the following scheme, the broad machine learning field is presented in a categorized way.

AI vs Machine Learning vs Deep Learning Presentation of all basic machine learning parts
Definition Machine Learning

In machine learning, algorithms are used to build statistical models based on training data. Roughly, these algorithms can be divided into three main learning techniques. While in supervised learning the result is predetermined by a cleanly labeled data set, unsupervised learning is completely self-organized. Here the patterns are to be explored independently.
In reinforcement learning, utility functions are to be independently approximated based on rewards received.

Machine Learning vs Deep Learning/ Deep Neural Learning

Deep learning is a subfield of machine learning similar to machine learning in Ai. Here, multilayer neural networks are used to analyze various factors in large amounts of data. These networks are similar to the human neural system. If you want to know more about this structure, read our article on perceptrons, the smallest unit of a neural network.
Optimization of neural weights, unlike machine learning, can be done using powerful GPUs. Pure machine learning is best used on structured data sets, while for unstructured data you should opt for deep learning. In the following graphic, we have summarized the main factors that make up deep learning. For the network types autoencoder and CNN we provide more detailed articles.

Representation of all basic deep learning components AI vs Machine Learning vs Deep Learning
Definition Deep Learning
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H2O AI – That’s why it’s so great

December 5, 2020 / / 0 Comments

There is a lot of Big Data software available now. One of them that you should definitely know about is the H2O AI Machine Learning solution.

With this open-source application you can implement algorithms from the fields of statistics, data mining and machine learning. The H2O AI Engine is based on the distributed file system Hadoop and is therefore more performant than other analysis tools. Your machine learning methods can thus be used as
parallelized methods.

Software Stack

They can program their algorithms in R, Python and Java and thus in the most important mathematical programming languages. H2O provides a REST interface to Python, R, JSON and Excel. Additionally, you can access H2O directly with Hadoop and Apache Spark. This makes integration into your data science workflow much easier. You already get approximate results while running the algorithms. A graphical web browser UI helps you to better analyze the processes and perform targeted optimizations.

How Clients Interacts with H2O AI

You can interact with H2O via clients using various interfaces. It is important for you to know that the data is usually not held in memory. They are localized in a H2O cluster and you only get a pointer to the data when you make a request.

How Clients Interacts with H2O AI
H2O Interaction flow

H2O Frame

The basic unit of data storage accessible to you is the H2O Frame. This corresponds to a two-dimensional, resizable and potentially heterogeneous data point. This tabular data structure also contains labeled axes.

H2O Cluster

Your H2O cluster consists of one or more nodes. A node corresponds to a JVM process and this process consists of three layers.

H2O Machine Learning Software Structure
H2O Software Stack

H2O Machine Learning Components

Language Layer

The R evaluation layer is a slave to the REST client front-end and in the Scala layer you can write native programs and algorithms. You can then use these with H2O Machine learning.

Algorithms Layer

This layer is where your algorithms are applied. You can run statistical methods, data import and machine learning here.

Core Layer

In this layer you handle the resource management. You can manage both the memory and the CPU processing capacity.

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5 Clustering Algorithms Data Scientists need to know – The key is always to understand the basic approach of any algorithm you want to use

November 22, 2020 / / 1 Comment

As a data scientist, you have several basic tools at your disposal, which you can also apply in combination to a data set. Here we present some clustering algorithms that you should definitely know and use

In times of Big Data, not only the sheer number of data increases, but also the relationships between them. More and more complex dependencies are formed. This makes it all the more difficult to recognize these similar properties and to assign the data to so-called clusters in a way that can be evaluated.

You have certainly heard of these algorithms and maybe used one or the other, but do you really know what clustering algorithms are?

What are clustering algorithms?

So let’s first clarify what these algorithms are in the first place. The goal is clear: You want to identify similar properties between individual data points in a data set and group them in a meaningful way. These properties are often high-dimensional.

With the help of cluster analysis, you want to reduce this high-dimensional information to a low-dimensional dependency. So, for example, a representation in 2D space. Clustering is an unsupervised machine learning technique and in the end you classify the data points by using algorithms.

The approach to clustering differs from technique to technique. All have their advantages and disadvantages, so it makes sense to try several on one set of data, or apply them in combination. Below we will introduce you to some popular clustering methods and explain their grouping approach.

This picture shows schematically popular Clustering Machine Learning Algorithms you should know as a data scientist
Clustering Machine Learning Algorithms – Popular clustering algorithms

Mean-Shift Clustering

The first algorithm we want to introduce you to is Mean-Shift Clustering. With this you can find dense areas of data points according to the concept of kernel density estimation (KDE). The basis of the clustering is a circular sliding window, which moves towards higher density at each iteration. Within the window, the centers of each class are determined, called centroids.

The movement is now created by moving the center to the average of the points within the window. The density within the sliding window is thus proportional to the number of points within it. This motion continues until there is no direction in which the motion can take more points within the kernel.

Clustering Machine Learning Algorithms - Schematic and simplified representation of the Mean-Shift principle.
Clustering Machine Learning Algorithms – Mean-Shift Clustering Priciple

Hierarchical Cluster Analysis (HCA)

With HCA, clusters are formed based on empirical similarity measures of the data points. This means that the two most similar objects are assigned one after the other until all objects are in one cluster. This results in a tree-like structure. In contrast to the K-means algorithm, which we will discuss later, similarities between the clusters play a role. These are represented by a cluster distance. With K-means, only all objects within a collection are similar to each other, while they are dissimilar to objects in other clusters.

You can create an HCA in different ways. There are two elementary procedures, the top-down and the bottom-up. If you want to know more about Hierarchical Cluster Analysis, read this article.

Schematic and simplified representation of the HCA clustering principle.
Clustering Machine Learning Algorithms – HCA Principle

Expectation-Maximization (EM) Clustering using Gaussian Mixture Models (GMM)

GMM basically assumes that the data points are Gaussian and not circular. The clusters are described by their mean and standard deviation. Each Gaussian distribution is randomly assigned to a single cluster and found using the Expectation-Maximization (EM) optimization algorithm. The probability of belonging to a cluster is then calculated for each data point. Thus, the closer a point is to the Gaussian center, the more likely it is then to belong to that cluster. Based on these probabilities, a new set of parameters for the Gaussian distributions is iteratively calculated. That is, the probabilities within a cluster are maximized.

K-Means clustering algorithms

The k-Means algorithm described by MacQueen, 1967 goes back to the methods described by Lloyd, 1957 and Forgy, 1965. You can use the algorithm besides cluster analysis also for vector quantization. Here, a data set is partitioned into k groups with equal variance.

The number of clusters must be specified in advance. Each disjoint cluster is described by the average of all contained samples. The so-called cluster centroid.


Each centroid is updated to represent the average of its constituent instances. This is done until the assignment of instances to the clusters does not
changes any more. If you want to learn more about the K-means algorithm, check this out.

Schematic and simplified representation.of the kmeans clustering algorithm
K-Means Principle

Density-Based Spatial Clustering of Applications with Noise (DBSCAN)

DBSCAN is a density-based cluster analysis with noise. From an arbitrary starting data point, neighborhood points are specified at a distance epsilon. Clustering then begins from a certain neighborhood data point count.

The current data point becomes the first point of the new cluster, or referred to as noise. In both cases, however, it is considered to be examined. The neighboring data points are then added to the cluster. Once all neighbors have been added, a new, unexamined point is called and processed. A new cluster is thus formed.

Schematic and simplified representation of the DBSCAN Clustering principle.
Clustering Machine Learning Algorithms – How DBSCAN works

The field of cluster algorithms is wide and everyone’s approach is different. You should be aware that there is no one solution. You have to consider each algorithm as another tool. Not every technique works equally well in every situation.

The key here is to always understand the basic approach of each algorithm you want to use. Build a small portfolio and get to know these techniques well. Once you master them, you should then add new ones. Knowing your own tools is crucial to avoid try and error and to gain control over your data. Remember: no result is a result. Your added value here is that even if an algorithm doesn’t work well on your data set, it will give you information about the data properties.

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What does HCA stand for?

November 13, 2020 / / 1 Comment

What does HCA stand for? What is the difference between Agglomerative and Divisive? When do I use the algorithm and what are its strengths? In this article we will clarify all these questions.

If you don’t know what clustering means, check out this article. Here we also explain four other clustering methods that you as a data scientist must know.

What is an HCA?

Hierarchical Cluster Analysis, or HCA, is a technique for optimal and compact connection of objects based on empirical similarity measures. The two most similar objects are assigned one after another until all objects are finally in one cluster. This then results in a tree-like structure.

What does HCA mean - This figure shows the basic principle of an applied HCA to raw data.
What does HCA stand for? Basic principle of an applied HCA to raw data.

So how does a hierarchical cluster procedure work?

Agglomerative vs Divisive Calculation

The basic clustering can be done in two opposite ways, Agglomerative and Divisive calculation.

Agglomerative clustering:

Agglomerative Nesting, abbreviated AGNES, is also known as the bottom-up method. This method first creates a cluster between two objects with high similarity, and then adds more clusters until all the data has been enclosed.

The divisive cluster calculation follows an opposite concept.

Divisive hierarchical clustering:

Divise Analysis, also known as DIANA, is a top-down method. All objects are directly framed into a cluster and then reduced in size.

In the following figure, the agglomerative process is compared with the divisive process.

What does HCA stand for? The figure compares the agglomerative and divisive calculation.
What does HCA stand for? Agglomerative vs Divisive Calculation

Thus, the goal is to represent the common properties in low dimension in multidimensional raw data. A strength of this machine learning method is the inclusion of cluster relationships. With K-means, only all objects within a collection are similar to each other, while they are dissimilar to objects in other clusters. If you want to know more about this other popular clustering method, read this article.

How to calculate the cluster distances?

As mentioned earlier, not only are similarities between data points in a cluster weighted, but also similarities between groups. These similarities are represented by distances between the clusters. These distances can be determined in different ways. The distance between the centroids of two clusters can be calculated. A single linkage is the shortest distance between two clusters, a complete linkage is the largest distance between two clusters and an average linkage is the average distance between two clusters.

The figure below contrasts each cluster distance calculation method.

The figure contrasts each cluster distance calculation method. A single linkage is the shortest distance between two clusters, a complete linkage is the largest distance between two clusters and an average linkage is the average distance between two clusters
Cluster distance calculation methods

In addition to the planar representation, the HCA can also be represented in a dendrogram.

HCA represented in a Dendrogram

Since an HCA describes a tree structure, it can be well represented in a dendrogram. Here the connections between the individual data elements and the connections between the clusters become well visible. This diagram can help to choose the optimal number of clusters in the data depending on where you intersect the tree.

In the following figure, for example, such a dendrogram is shown in dependence on Agglomerative and Divisive Calculation.

The figure shows a HCA represented as a dendrogram in dependence to Agglomerative and Divisive Calculation.
HCA presented as dendrogram in dependence to Agglomerative and Divisive Calculation.
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That is why Liquid State Machines (LSM) are great

October 31, 2020 / / 0 Comments

– Recently developed computational model

– does not require information to be stored in some stable state of the system

→ the inherent dynamics of the system are used by a memory less readout function to compute the output

→ can be used for complex Tasks (pattern classification, function approximation, object tracking, …)

– LSMs take the temporal aspect of the input into account

Concept

The figure shows a typical structure of a liquid State Machine.
Liquid State Machine

Reservoir/ Liquid

– large accumulation of recurrent interacting nodes
→ is stimulated by the input layer
– Liquid itself is not trained, but randomly constructed with the help of heuristics
– Loops cause a short-term memory effect
– preferably a Spiking Neural Network (SNNs)
→ are closer to biological neural networks than the multilayer Perceptron
→ can be any type of network that has sufficient internal dynamics

Running State

→ will be extracted by the readout function

– depend on the input streams they’ve been presented

Readout Function

– converts the high-dimensional state into the output

– since the readout function is separated from the liquid, several readout functions can be used with the same liquid

→ so different tasks can be performed with the same input

lsm readout fcts
different types of readout functions
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AutoEncoder – What Is It? And What Is It Used For?

October 29, 2020 / / 0 Comments

AutoEncoder – In data science, we often encounter multidimensional data relationships. Understanding and representing these is often not straightforward. But how do you effectively reduce the dimension without reducing the information content?

Unsupervised dimension reduction

One possibility is offered by unsupervised machine learning algorithms, which aim to code high-dimensional data as effectively as possible in a low-dimensional way.
If you don’t know the difference between unsupervised, supervised and reinforcement learning, check out this article we wrote on the topic.

What is an AutoEncoder?

The AutoEncoder is an artificial neural network that is used to unsupervised reduce the data dimensions.
The network usually consists of three or more layers. The gradient calculation is usually done with a backpropagation algorithm. The network thus corresponds to a feedforward network that is fully interconnected layer by layer.

Types

AutoEncoder types are many. The following table lists the most common variations.

The figure shows all common AutoEncoder types
AutoEncoder types

However, the basic structure of all variations is the same for all types.

Basic Structure

Each AutoEncoder is characterized by an encoding and a decoding side, which are connected by a bottleneck, a much smaller hidden layer.

The following figure shows the basic network structure.

The figure shows the basic AutoEncoder structure.
AutoEncoder model architecture


During encoding, the dimension of the input information is reduced. The average value of the information is passed on and the information is compressed in such a way.
In the decoding part, the compressed information is to be used to reconstruct the original data. For this purpose, the weights are then adjusted via backpropagation.
In the output layer, each neuron then has the same meaning as the corresponding neuron in the input layer.

Autoencoder vs Restricted Boltzmann Machine (RBM)

Restricted Boltzmann Machines are also based on a similar idea. These are undirected graphical models useful for dimensionality reduction, classification, regression, collaborative filtering, and feature learning. However, these take a stochastic approach. Thus, stochastic units with a particular distribution are used instead of the deterministic distribution.


RBMs are designed to find the connections between visible and hidden random variables. How does the training work?
The hidden biases generate the activations during forward traversal and the visible layer biases generate learning of the reconstruction during backward traversal.

Pretraining

Since the random initialization of weights in neural networks at the beginning of training is not always optimal, it makes sense to pre-train. The task of training is to minimize an error or a reconstruction in order to find the most efficient compact representation for input data.

The figure shows the pretraining procedure of an autoencoder according to Hinton.
Training Stacked Autoencoder


The method was developed by Geoffrey Hinton and is primarily for training complex autoencoders. Here, the neighboring layers are treated as a Restricted Boltzmann Machine. Thus, a good approximation is achieved and fine-tuning is done with a backpropagation.

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