1. Today’s topics Strength of Weak Ties Next Topic Acknowledgements
How does Google rank webpages in search?
Acknowledgements
James Moody, Alan Kirman, Dejan Vinkovic

2. Factors influencing diffusion
Network structure (unweighted)
density
degree distribution
clustering
connected components
community structure
Strength of ties (weighted)
frequency of communication
strength of influence
Spreading agent
attractiveness and specificity of information

3. How does strength of a tie influence diffusion?
M. S. Granovetter: The Strength of Weak Ties, AJS, 1973:
Finding a job through a contact that one saw
frequently (2+ times/week) 16.7%
occasionally (more than once a year but < 2x week) 55.6%
rarely 27.8%
But… length of path is short
contact directly works for/is the employer
or is connected directly to employer

4. Strength of Weak Ties
Why do leads for new jobs come from weak contacts?
What binds communities together?
How do ties afffect access to resources?
What are the social implications?

5. strong ties are likely to “close”
A strong tie
frequent contact
affinity
many mutual contacts
Less likely to be a bridge (or a local bridge)
“forbidden triad”:
strong ties are likely to “close”
Source: Granovetter, M. (1973). "The Strength of Weak Ties", American Journal of Sociology, Vol. 78, Issue 6, May 1973, pp

6. Triadic Closure

7. Triadic Closure

8. Triadic Closure

9. Strength of ties on facebook

10. Strength of ties on Facebook
Why are some ties more common than others?

11. Strength of ties on twitter
Study by Wu, Golder & Huberman

12. What indicates cohesion?
Mutuality of ties
everybody in the group knows everybody else
Closeness or reachability of subgroup members
individuals are separated by at most n hops
Frequency of ties
Among members
everybody in the group has links to at least k others
Among subgroup members compared to nonmembers
Why?
Discover communities of practice
Measure isolation of groups
Threshold processes:
I will adopt an innovation if some number of my contacts do
I will vote for a measure if a fraction of my contacts do

13. Columbia Small World Experiment
Identical protocol to Travers and Milgram, but conducted via the Internet
60,000 participants from 170 countries attempting to reach 18 different targets
Results
Median “true” chain length 5 < L < 7
Successful chains disproportionately used
professional ties (34% vs. 13%)
ties originating at work/college
target's work (65% vs. 40%)
weak ties (Granovetter)
. . . and disproportionately avoided
hubs (8% vs. 1%) (+ no evidence of funnels)
family/friendship ties (60% vs. 83%)