Small worlds

Human progress is neither automatic nor inevitable… Every step toward the goal of justice requires sacrifice, suffering, and struggle; the tireless exertions and passionate concern of dedicated individuals.

Martin Luther King, Jr.


  • Tech policy
  • Designated reminder
  • Proposed structure for class
    • Semi-“flipped” class

Questions from the homework

  • Edgelists
  • Matrices
  • Graphs

Why is the network perspective so important?

Power failure example

Complex systems are all around us

  • Interdependent
  • Non-linear
  • Unpredictable, even when individuals are understood

Segregation Example

  • Link to this example is on the wiki
  • Go through this with a partner
  • Discuss in small groups
  • Discuss as a class


  • How does individual behavior aggregate?
  • When is this simple? When is it complicated? (TPS)
  • Fire example
  • Boids example

Interdependence and Structure

  • If components are interdependent, then how they are connected matters
    • Both for individuals and for the system
  • What are some examples of this? (TPS)

Structure of networks

  • What do real social networks look like?
  • What is clustering?
  • What is a random graph?

Random Graph

Actual social network

  • Faculty friendship network

Other insights and ideas from Six Degrees?


  • Read Chapter 3 of Six Degrees

Six Degrees of Wikipedia

Concepts from reading

  • Structure and agency
  • Clustering
  • Path lengths
  • Distance

Putting it together

  • What does “Six Degrees” refer to?
  • What is the puzzle?
  • What is the solution?

Milgram Experiment

  • What was he trying to do?
  • Did you find the results surprising?
  • Do you think that technology has changed this today? In what ways
  • What do you think the distance is of the Purdue network?

How is knowledge connected?

  • Six Degrees of Wikipedia
    • Can be done in pairs, turn in with both names on it


  • Find someone with HI driver’s license