CS6010 Social Network Analysis Unit V
- 1. CS6010 - Social Network Analysis Unit V – Visualization and Applications of Social Networks Kaviya.P AP/IT Kamaraj College of Engineering & Technology 1
- 2. Unit V – Visualization and Applications of Social Networks Graph theory - Centrality - Clustering - Node-Edge Diagrams - Matrix representation - Visualizing online social networks, Visualizing social networks with matrix-based representations - Matrix and Node-Link Diagrams - Hybrid representations - Applications - Cover networks - Community welfare - Collaboration networks - Co-Citation networks. 2
- 3. Graph theory - Centrality - Clustering 3
- 4. Graph Theory Social Network Analysis • Social networks are built when actors belonging to different social groups are connected to each other. • Sociograms used to represent social networks and started to explore the social relations in a formal study. Graph Theory • Graph theory is the study of graphs, which are mathematical structures used to model pair wise relations between objects. • Many fundamental concepts and metrics in social network analysis are derived from graph theory, because graph theory formally represents social networks with structural properties. 4
- 5. Graph Theory – Fundamental Definitions • Node Degree The degree of a node in a graph, is the number of edges incident to the node. It can be calculated using the formula, Undirected Graph: N*(N-1)/2 Directed Graph: N*(N-1) where N is Number of nodes present in the graph. • Node Density It is a graph in which the number of edges is close to the maximal number of edges (which is present in the actual graph). Undirected Graph: (2*E/N)*N-1 Directed Graph: (E/N)*N-1 where E is Number of edges in the network and N is Number of nodes in the network 5
- 6. Graph Theory – Fundamental Definitions • Path Length – The path length can be defined as the distances between pairs of nodes in a network graph. – Average path length is the average of these distances between all pairs of nodes. • Component Size – Component size is counted by the number of connected nodes in a graph. – A graph is connected if all pairs of nodes are reachable, and for each pair of two nodes, one of them is reachable from the other. – On the other hand, if a graph is not connected, the graph can be partitioned into several connected subgraphs where each component size can be calculated by the number of connected nodes in each subgraph. 6
- 7. Centrality • Centrality – Centrality is a measure indicating the importance of node in the network. – The measure of centrality is thus used to give a rough indication of the social power of a node based on how well they connect the network. – HITS and PageRank are two most famous representatives using centrality for ranking. – It is further divided into: Degree Centrality, Betweeness Centrality, and Closeness Centrality 7
- 8. Centrality • Degree Centrality – It is defined as the number of edges incident upon a node and thus it is usually the first way to calculate the nodes that are most potential to determine other nodes which is present in the network. – How many people can reach this person directly? • Betweeness Centrality – It is to measure the connectivity of the neighbor's of your node and to give a higher values for nodes which bridge clusters. – How likely is this person to be the most direct route between two people in the network? • Closeness Centrality – How distinct a node is to the other nodes in the network? – How fast can this person reach everyone in the network? 8
- 9. Clustering • Clustering – Also called community, it refers to a group of nodes having denser relations with each other than with the rest of the network. – It is to measure the degree's of nodes to decide which nodes in a graph tends to be clustered together. – Clustering for a single vertex can be measured by the actual number of the edges between the neighbors of a vertex divided by the possible number of edges between the neighbors. • Clustering coefficient Clustering coefficient measure is to quantify how close its neighbor's are to being a complete graph. Clustering coefficient = 3 * TC / CT TC - Triad Closure CT - Connected Triple The clustering coefficient of tree is zero, which is easy to see if we consider that there are no triangles of edges (triads) in the graph. 9
- 10. Node-Edge Diagrams - Matrix Representation 10
- 11. Node – Edge Diagrams • A node-edge diagram is an intuitive way to visualize social networks. • With the node-edge visualization, many network analysis tasks, such as component size calculation, centrality analysis, and pattern sketching, can be better presented in a more straightforward manner. • There are three kinds of layouts: – Random layout – Force – directed layout – Tree layout Random Layout • Random layout is to put the nodes at random geometric locations in the graph. • It does not provide very clear visualization results, particularly when the number of nodes immensely increases, e.g. more than thousands of nodes • Since a random layout algorithm can efficiently draw the social network graph in linear time, O(N). 11
- 12. Node – Edge Diagrams Force-Directed Layout • Also known as a spring layout, this simulates the graph as a virtual physical system. • The edges act as spring and the nodes act as repelling objects. • There exists gravitational attraction or magnetic repulsion between each node in the graph. • Generally, an initial random layout will be yielded first, and then the force- directed algorithms will run iteratively to adjust the positions of nodes until all graph nodes and attractive forces between the adjacent nodes run to convergence. • Since a force-directed layout may take hundreds of iterations to obtain a stable layout, the running time is at least O(N log N) or O(E), where N is the number of nodes and E is the number of edges. • The running cost of a force-directed layout is much higher than that of a random layout, especially when the number of nodes is large. It is therefore not suitable for graphs larger than hundreds of nodes. 12
- 13. Node – Edge Diagrams Tree Layout • A basic tree layout is to choose a node as the root of tree, and the nodes connected to the root become children of the root node. • Nodes that are at more levels away from the root become the grand-children of the root and so on. • It can display a more structural layout than graph layouts by considering more contextual information. • Because of the hierarchical nature of a tree layout, trees are more straightforward to grasp human eye than general graphs. • For a better visual presentation of domain specific information, more suitable variants of the tree layout were proposed, such as hyperbolic tree layout and a radial tree layout. • The tree visualizations utilize the idea of focus + context to better the visualization effects with animation techniques and help users to obtain both global and local views of a social network in a 2D display. 13
- 14. Matrix Representation Matrix Representation • A matrix presentation can help minimize the occlusion problems caused by the node-edge diagram. • The matrix-based representation of graphs offers an alternative to the traditional node-edge diagrams. • With a matrix-based representation, clusters and associations among the nodes can also be better discovered when the number of nodes increases. • When the relationships are complex, a matrix-based representation can effectively outperform a node-edge diagram in readability since the high connectivity of a node- edge representation will easily diffuse the focus. • An enhanced matrix-based representation, called MatrixExplorer, was developed in 2006 to visualize social networks with a Dual-Representation. • MatrixExplorer can provide users with two synchronized representations of the same network: matrix and node-edge. • A matrix-based visualization may not entirely replace a conventional node edge diagram, yet it could complement the shortcomings of a node-edge diagram to better the social network visualization. 14
- 15. Visualizing Online Social Networks 15
- 16. Visualizing Online Social Networks • Visualizations of online social networks were developed according to the attributes of network sociality to present their network structure. • Visualization of online social networks can be analyzed according to their attributes of sociality, including Web communities, email groups, digital libraries, and Web 2.0 services. 16
- 17. Visualizing Online Social Networks Web Communities • Different social network services were created on the Web to help people maintain their social relationships. • SixDegrees.com - Web-based dating services have been established to provide people more convenient ways to build up their social relationships and communities. • Club Nexus - Friendship network data of Stanford students and allowed them to explicitly list their friends by their profiles. Students registered on Club Nexus were provided with the profiles of their year in school, major, residence, gender, personalities, hobbies and interests to facilitate interacting with their online social networks. • Vizster - Node-edge network layouts for exploring connectivity in large graph structures, supporting visual search and analysis, and automatically identifying and visualizing community structures • FOAF - To visualize such human-centric social relationships based on Semantic Web social metadata. • Entity Cube - Help people discover real-world entities, such as people, locations, and organizations, and explore their social relationships. 17
- 18. Visualizing Online Social Networks Email Groups • Email service is one of the most popular applications that people often use to connect each other and deliver messages in their daily lives. • Soylent - To study the social patterns and the temporal rhythms of daily email activities. Through the Soylent visualization, mutual interactions between different users and groups, and their everyday collaboration activities can be clearly displayed. • Social Network Fragments (SNF) and PostHistory - To visualize the major two dimensions of email activities: people and time. • In SNF, different colors are utilized to indicate people from different contexts of ego’s social life. • PostHistory presents social network visualization with a calendar panel on the left and a contacts panel on the right. The email exchange activities with time progress can thus be visualized in an interactive calendar-like interface. • Through the analysis of email content, the social relations and mutual interactions between a user and her contacts can be clear presented in the mail. 18
- 19. Visualizing Online Social Networks Digital Libraries • In digital libraries, social networks can be mainly analyzed from two aspects: authors and writings. • Co-Authorship Networks - co-authorships can be mined from the existing publications and organize the co-authorship networks. • From the visualization of the co-authorship networks a small world graph can be drawn with the connections of authors from different places in the world. • Co-Citation Relations - Social networks in digital libraries can be discovered from the citations and co-citations among writings themselves. – Circle View - To visualize academic citation relations with interactive design and highlighted color. – FP-tree - To present co-citation network from a new perspective, namely, visualizing social networks based on a paper-reference matrix instead of using a reference-reference matrix. 19
- 20. Visualizing Online Social Networks Web 2.0 Services • Web 2.0 applications are popularly accessed by users to connect their social networks, such as Twitter and Facebook. • Nexus4 is a visualization application on Facebook communities to illustrate their large network graphs. • TouchGraph is a modern Facebook visualization that provides users with interactive functionalities to help access social networks. • IRNet visualizes interpersonal relationships in social networks with focus + context techniques to present both local and global views. 20
- 21. Visualizing Social Networks with Matrix- Based Representations - Matrix and Node- Link Diagrams 21
- 22. Visualizing Social Networks with Matrix- Based Representations • Matrices or node-link diagrams both have advantages and drawbacks for visualizing social networks. • Matrix and node-link diagrams have different properties making them suitable representations for different tasks and datasets. • Node-link diagrams are more effective for very small (under 20 vertices) and sparse networks whereas Matrices outperform well in dense networks. The advantages of matrices: • Matrices provide powerful overview visualization since the time to create them is low and since they are always readable. • Matrices do not suffer from node overlapping. • Matrices do not suffer from link crossing each other; therefore they are a viable alternative for dense networks. • Matrices show all possible pairs of vertices, they can highlight the lack of connections and also the directedness of the connections. They are particularly appropriate for directed and dense networks. 22
- 23. Visualizing Social Networks with Matrix- Based Representations The advantages of node-link diagrams: • These representations are familiar to a wide audience; they constitute a powerful communication tool. • For small or sparse networks, node-link diagrams were more effective than matrices. • The space used by matrices is larger than the space to display node-link diagrams. Therefore, for a compact representation, node-link diagrams are a better choice. • When the analysis requires to perform a number of path-related tasks (e.g. find the shortest path from John to Mary), node-link diagrams are more appropriate. 23
- 24. Visualizing Social Networks with Matrix- Based Representations Matrix + Node-Link Diagrams • Matrix Explorer designed to combine advantages of both representations and to support the visual exploration of social networks. • Following are the steps to combine matrices and node-link diagrams. – Initiate the exploration – Explore interactively and iteratively – Find a consensus in the data or validate an hypothesis – Present the findings 1. Initiate the Exploration • Readable representation and low rendering time of matrices provide suitable representations to initiate the exploration. • Persons are nodes or rows/columns, email exchanges between two persons are represented by a link or a cell filled with black in the matrix. • From matrix representation, Each black dot represents a connection between a row and a column (i.e. an email exchange between two persons); the gray background shows the lack of connection. • The node-link representation, using a traditional force-directed layout, makes it difficult to identify specific nodes or links. 24
- 25. Visualizing Social Networks with Matrix- Based Representations 2. Explore interactively and iteratively • Both the matrix and node-link representations support the analysis of the network at different levels of details. • For instance, if an analyst is looking for an overview of the network to identify its main communities, the matrix is the best option. • To identify actors bridging two communities for example, node-link diagrams constitute a better alternative. • Matrix Explorer can provide multiple views of the network and provide a number of tools to interactively manipulate matrix and node-link representations. • Set of tools available for manipulating matrix and node link representations are listed below: – Interactive specification of visual attributes – Interactive layout and reordering – Automatic layout and reordering techniques – Computer-assisted layout and reordering techniques – Interactive filtering – Interactive clustering – Overview + Detail techniques to navigate in both representations 25
- 26. Visualizing Social Networks with Matrix- Based Representations 3. Find a Consensus in the Data • Different techniques to reorder the matrix may lead to different cluster sets. To help analysts find a consensus and validate hypotheses, some support is needed. • MatrixExplorer allows analysts to find consensus in the data through simple interactions. • For example, by associating visual variables such as colors to different cluster sets and by reordering the matrix several times, analysts can identify clusters appearing clearly in multiple orders as more valid. • Matrix Explorer also provides a technique to indicate the degree of membership of the element to a given cluster. 4. Present Finding • Matrix representations may prove effective when exploring large networks. • Node-link diagrams are essential to communicate findings to a wide audience. • Matrix Explorer allows users to generate pictures while performing the exploration. 26
- 28. Hybrid Representation • Matrix Explorer Providing both matrix and node-link diagrams to the user has a number of advantages but also drawbacks: – It requires a large amount of display space. – At least two display monitors are required to comfortably use Matrix Explorer. – Switching from one representation to the other may induce high cognitive load to the user. • Two hybrid representations were developed namely, – MatLink and NodeTrix Augmenting Matrices – The principle of MatLink is to augment a standard matrix representation with links on its borders. – These links provides a dual encoding of the connections between actors. Two types of links are added to the representations: static links and interactive links. – When a row or column is selected, these links show a shortest path to any other row or column placed under the cursor. 28
- 29. Hybrid Representation • Assessing the Readability of MatLink – MatLink introduced specific tasks of social network analysis: find a cut point, find a clique and find communities (strongly connected groups). – Node-link diagrams performs better in the identification of cut points. With MatLink, this task requires to identify specific visual patterns of the links. • Using MatLink for Navigating in the Matrix – To improve readability of matrices, Matlink supports navigation. Since matrices display actors in rows and columns, they require far more space than node-link diagrams to represent a network. – Three techniques that provide users with effective tools to navigate in large matrices with MatLink were listed below: • Melange: Folds the space between two far away. Users may see side by side parts of the matrix that are far away. • Bring-and-go: Neighbors of an actor closer as if their links were elastic, by moving the cursor over one of the neighbor and releasing the mouse, the view and the node travel to its previous location. • Link Sliding: Allows users to locks their cursor to a given link and travel very fast to its destination. 29
- 30. Hybrid Representation Merging Matrix and Node-Link Diagram • Node-link diagrams or matrices perform differently according to the types of visualized networks. • NodeTrix is a hybrid visualization merging node-link diagrams and matrices. • The principle of NodeTrix is to represent the global network as a node-link diagram and the locally dense subparts as matrices. • Interactive Exploration – NodeTrix developed a number of interactions based on traditional drag-and- drop of objects with the mouse cursor for ease creation, exploration and edition of matrices. – Matrix representations have the advantage of placing actors of the network linearly (in rows and in columns), thus it becomes easy to identify the community members connected to external actors. – Drawback: Making it impossible to place an actor in two different communities. • Presenting Finding – NodeTrix can be used for both exploration and communication because matrices can be expanded showing detailed information on actors and connections showing higher-level connection patterns. 30
- 31. Applications of Social Network Analysis 31
- 32. Applications of Social Network Analysis • Businesses use SNA to analyze and improve communication flow in their organization, or with their networks of partners and customers. • Law enforcement agencies (and the army) use SNA to identify criminal and terrorist networks from traces of communication that they collect; and then identify key players in these networks. • Social Network Sites like Facebook use basic elements of SNA to identify and recommend potential friends based on friends-of-friends. • Civil society organizations use SNA to uncover conflicts of interest in hidden connections between government bodies, lobbies and businesses. • Network operators (telephony, cable, mobile) use SNA-like methods to optimize the structure and capacity of their networks. 32
- 33. Applications of Social Network Analysis 33
- 34. Applications of Social Network Analysis Organizational Issues • Organizational Behavior is the study and application of knowledge about how people, individuals, and groups act within an organization. • In any organization, cooperation and information sharing among the workers is very important for the success of an organization. • SNA can also be used to identify the key or central persons of an organization which also helps to understand important to go people in an organization. – Team Formation – Improved Information sharing – Identifying bottlenecks – Hidden barriers 34
- 35. Applications of Social Network Analysis Recommendation and E-commerce Systems • Recommendation systems are web services that provide information about entertainment elements, scientific papers, books, fashion elements such as clothing etc. • Typically recommendation systems allow users to select and rate items according their own interest and opinion. • Allowing users to create their own list of items according to their own likes, and allowing user to create their favorites. • Most of the e-commerce sites such as Amazon, ebay etc. have their own recommendation systems for recommending customized products to customers and also tries to improve targeted marketing of products. 35
- 36. Applications of Social Network Analysis Recommendation and E-commerce Systems • Social Network analysis in recommendation systems helps to enhance selling by converting browsers into buyers. Also, these websites acts as recommender agents to learn customers, obtain their preference and provide items of their interest. • The SNA makes use of various metrics such as centrality, cohesiveness, degree of vertex etc. each may reflect different meaning in recommendation system analysis. – Node with high centrality means it has high impact on other nodes. – The vertex similarity may be considered as metric to search the individuals having same interest or preference. – Cohesiveness property of network defines a group of nodes of network bounded with each other by some relation and may have common characteristics. 36
- 37. Applications of Social Network Analysis Covert Networks • The covert networks are hidden, the actors of such network does not disclose their information to the external world. • Covert groups have cellular networks structure which is different from hierarchical organizations. The terrorist and criminal networks are good examples of such networks. • SNA has been successfully applied to such domains to understand covert cell operations and their organization. • SNA is applied on terrorism database for predicting node and link, discovering interesting patterns and actors involved in an event. • Another vital application of SNA for terrorist database is to predict terrorism activities. 37
- 38. Applications of Social Network Analysis Covert Networks • SNA tools has been used to identify these organization structures and provide critical information for terrorist detection and terrorism prediction. • Centrality is the most important and widely used measure in SNA used to identify key players in terrorist network. • To facilitate this, the regular day-to-day activities of the key players are monitored. The hidden actors are discovered by monitoring contact and the extend of contacts of known terrorists with other people. 38
- 39. Applications of Social Network Analysis Web Applications • Web is being used by different community for various purposes such academic improvement, knowledge sharing, interest sharing, communication and profiling, research, business etc. Hence, different techniques are required to improve and optimize the usability of web. • SNA models web as a graph where web pages are represented as nodes and hyperlinks as edges. • Node similarity based SNA techniques are employed to classify the web based on its usage and contents in order to understand the scope of domain and density. • SNA is also used in search engines such as google to enhance keyword search quality. Google uses PageRank as a measure of popularity, which is obtained by simulating a random walk on network of web pages and computing prestige of web pages. 39
- 40. Applications of Social Network Analysis Community Welfare • SNA is used to analyze different types of relations such as communication patterns, physical contacts, sexual relationship etc. • SNA may reveal the patterns of human contact which may lead to spread of disease such as HIV in population. • SNA to examine and observe farm animal network to identify patterns of disease spread from one animal to another. • Mass surveillance is done with the purpose of protecting people from criminals, terrorists or political subversives to maintain social control. • Social Networks which are made for strengthening community resilience against disasters (natural or human-made) can reveal vulnerabilities within a network. 40
- 41. Applications of Social Network Analysis Collaboration Networks • Collaboration network consists of groups of persons working together to perform particular activity and studying human collaboration is an important topic in sociology. • The widely studied collaboration network by researchers in context of SNs are Co- authorship collaboration network and movie actor collaboration networks. • The co-authorship network is – To study dynamics in patterns of interactions between educational entities or communities. – To study and understand the interdisciplinary research which is key factor for innovation. • Another type of collaboration network is knowledge collaboration network. The information about Open Source Software needs to be distributed amongst community or users. 41
- 42. Applications of Social Network Analysis Co-Citation Networks • Co-citation is used as a measure of similarity between two objects. • Co-citation analysis helps to understand the status and structure of scientific research. • Basic two approaches of co-citation are – Author co-citation and – Document co-citation • In the field of methodological evaluation, co-citation analysis has been employed to search for invisible colleges. • This reveals the research network consisting of different institutions linked to each other informally by having indicators to each others documents/papers which can be used to get group of institutes having similar ongoing research. 42