 Building Networks

First Decisions  What do the nodes represent?  What do the edges represent?  Know this before doing anything with data!

Examples  Facebook network  What are the nodes? Edges?  What can you see if you were able to visualize this network?  What would network statistics mean in this network?  What patterns might emerge?

Networks with multiple node types  Bipartite graphs have two node types that do not have connections within the type  E.g. no people connected to one another  Graphs can have multiple node types and not be bipartite

To Build a Network  Step 1: Define Nodes  What are they?  What are the criteria for being included?  Example: Make a network of characters in your favorite TV show. Nodes are characters, but which ones?  Main characters?  Extras?  People mentioned who never appear?  Something else?

To Build a Network  Step 2: Define Edges  What does an edge represent?  What is the criteria for adding one?  Same example: If two characters know one another, we add an edge.  What if they only know of one another?  What if they obviously know one another but never interact on the show?

Handling Large Networks  Some networks may be too big to analyze.  Filtering or sampling will be important

Snowball Sampling  When working with a large network, choose a starting node  Get that node, its connections, their connections, and so on until the network is the right size for analysis  Problems: Biased toward the part of the network sampled, may miss other features  Benefits: Easy to do, common

Random Sampling  Randomly select a certain percentage of nodes and keep all edges between them OR Randomly select a certain percentage of edges and keep all nodes that are mentioned.  Problems  Edge sampling biased toward high degree nodes  Node sampling loses some structural characteristics  Benefits  Easy  Node sampling keeps some network statistical features

Egocentric Network Analysis  Instead of looking at the whole network, look at the egocentric networks of some nodes.  A different type of analysis than overall network analysis, but it shows the role of an individual in context.

Exercise  Read the first chapter of Pride and Prejudice  Draw a network of the characters with their relationships.