By: Dyrol Lumbard, Mathematical Institute, University of Oxford
People make a city. Each city is as unique as the combination of its inhabitants. Currently, cities are generally categorised by size, but research by Oxford Mathematicians Peter Grindrod and Tamsin Lee on the social networks of different cities shows that City A, which is twice the size of City B, may not necessarily be accurately represented as an amalgamation of two City Bs.
The researchers use Twitter data from ten different UK cities, showing reciprocal tweets within each city. By defining cities in terms of these social network structures, they break each city into its comprising modular communities. Next, they build virtual cities from the actual cities. For example, Bristol has 74 communities. Randomly sampling (with replacement) from these communities 145 times builds a virtual city the same size as Manchester – but made up of modular communities actually observed in Bristol. How much does our virtual Manchester network resemble the true Manchester network? The answer is very closely. So if one was trying to spread a message via Twitter through Manchester, or make other social interventions, it may prove beneficial to test the same activity in Bristol first.
However, sampling the Bristol communities to create a virtual city the same size as Leeds, which is smaller than Manchester, does not create a network of similar structure to the ‘real’ Leeds. This highlights that the relationship between social structures of cities is not immediately obvious, and requires further analysis. Furthermore, this relationship is not symmetrical: a virtual city created by randomly sampling 74 communities from the Leeds network, does in fact resemble the true Bristol social network. So Bristol could learn from Leeds but not vice versa.
In summary, we may sometimes replicate one city using the communities from another. However, some cities have a very diverse range of communities, making them difficult to replicate – Leeds is a good example of this. Perhaps cities can be put into classes where those cities in the same class are socially similar and so any experience of social phenomena or reactions to interventions in one such city may be relevant to another.
This article was first published on the website of the Mathematical Institute, University of Oxford, on 11 March 2016.