»»fiµµSmall World NetworksInteresting Small World Problem • After Milgram, not much done for 30 yearsis therefore:– Theory impossible with pencil and paper– Experiments are hard to perform• How is it possible for Social Networks to be:– Large-scale network data hard to collect– Very highly ordered locally (like social groups), and• Arrival of modern computers and the – Still be “small” globally? (like random networks)Internet enabled new approaches• Using simulation approach, Watts and • Problem is that Structure makes Analysis HardStrogatz (1998) asked: what are the – It was theoretical difficulty that led to Milgram’sconditions required for any network to beexperimental approach in the first place1. Locally “ordered” and2. Globally “small”?Path Length (L) Rewiring networks from and Clustering (C)Order to Randomnessp = 0 (Ordered) p = 1 (Random)n lnnL • “Large” L • “Small”k ln k3 k• “High” • “Low”C C 0p = 0 p = 1 4 nIntuition: the world can be Increasing randomness either “large and highly clustered”, or “small and poorly clustered”, but not “small and highly clustered”Path Length L(p) and Clustering C(p)Small-World Networksversus Random Rewiring (p)normalized by their values at p=0• Main result:– For large N, a small fraction (p) of shortcuts will 1 contract (global) Length, but leave (local) Clustering unchanged.0.8 • Required conditions are trivialC(p)/C(0)– Some source of “order”0.6– Some source of randomness 0.4 • ...