Centrality | EPFL Graph Search

In graph theory and network analysis, indicators of centrality assign numbers or rankings to nodes within a graph corresponding to their network position. Applications include identifying the most influential person(s) in a social network, key infrastructure nodes in the Internet or urban networks, super-spreaders of disease, and brain networks. Centrality concepts were first developed in social network analysis, and many of the terms used to measure centrality reflect their sociological origin. Definition and characterization of centrality indices Centrality indices are answers to the question "What characterizes an important vertex?" The answer is given in terms of a real-valued function on the vertices of a graph, where the values produced are expected to provide a ranking which identifies the most important nodes. The word "importance" has a wide number of meanings, leading to many different definitions of centrality. Two categorization schemes have been proposed. "Imp

A network‐based approach to predict commercial activities optimal location and infer urban centrality indices

Jan Stauffer

The goal of this master thesis is to analyze the structure of the commercial activities in the cities of Geneva and Barcelona. This study implemented a network‐based approach to predict the location of commercial activities, on the basis of which a tentative inference of urban centrality indices was carried out. This was done by using a ‘grouping algorithm’ and by calculating a ‘location quality index’, both presented and described in the ‘Network‐based predictions of retail store commercial categories and optimal locations’ paper published by Pablo Jensen (2006). In the first part of the work, an existing prototype of the algorithm – implemented during a semester project as VBscript in the GIS software ‘Manifold’ – was optimized. Time gains of a factor 10 for the ‘link computation’ and more stable results for the ‘grouping algorithm’ could be achieved. A spatial representation of the grouping‐results was elaborated and permitted to recognize the dominance areas of the activity‐groups for both cities. The maps created with the ‘location quality index’ (Q‐Index) allowed to identify preference areas and location patterns for the different categories of commercial activities. It was possible to notice that the initial definition or choice of the categories is very important to get clear results. Indeed, the results on Geneva with only 18 categories performed less well than the ones with 45 and 48 categories on Geneva and Barcelona respectively. Furthermore, tests were carried out on the basis of hectometric referenced data, like the database of the Swiss federal census of enterprises. The results obtained are very similar to the ones from the postal address‐based data (precision 2‐3m). This shows that the algorithm used here could be applied to whole Switzerland in a possible future study to determine the overall structure of commercial activities in the country, to identify possible central locations, or also to propose it as useful tool for cantonal or regional economic development agencies to determine optimal locations for new coming large foreign industrial commercial groups. The results of the comparisons between the Q‐Index and centrality indices showed that an approximation of the latter by the first one is not possible. But the Q‐Index (describing location preference per category of commercial activity) can be used as a complementary indication to centrality (which describes accessibility in a broad sense). As a perspective, it would be particularly interesting to work on a definition of a Q‐Index for groups of categories or even of a global integrated one: it would then be probably possible to use this index as a surrogate for the evaluation of value landed property in particular.

2009

Scale-dependent measure of network centrality from diffusion dynamics

Alexis Arnaudon

Classic measures of graph centrality capture distinct aspects of node importance, from the local (e.g., degree) to the global (e.g., closeness). Here we exploit the connection between diffusion and geometry to introduce a multiscale centrality measure. A node is defined to be central if it breaks the metricity of the diffusion as a consequence of the effective boundaries and inhomogeneities in the graph. Our measure is naturally multiscale, as it is computed relative to graph neighborhoods within the varying time horizon of the diffusion. We find that the centrality of nodes can differ widely at different scales. In particular, our measure correlates with degree (i.e., hubs) at small scales and with closeness (i.e., bridges) at large scales, and also reveals the existence of multicentric structures in complex networks. By examining centrality across scales, our measure thus provides an evaluation of node importance relative to local and global processes on the network.

AMER PHYSICAL SOC2020

Innovation dynamics in the medical device sector

Monica Coffano

This thesis aims to provide new insights into the evolution of the medical device sector (MedTech). After the analysis of the history of the sector, I examine the key points that in the past 60 years, have led the industry to grow so impressively, and I proceed to an analysis of the actual situation. The scope of the thesis is to understand if what has stimulated the success of the sector at the beginning is still important, and if the introduction of new elements has positively changed the evolution of the sector. The thesis is composed of three works. The first work (chapter 2) is developed in collaboration with Dominique Foray and Michele Pezzoni. I analyze how the network structure of inventors in the Swiss regions can influence regional innovation performance. I aim to contribute to the existing literature related to the debate on the importance of inventors' collocation for the creation of innovation. I claim that an increased degree centrality of MedTech inventors in the regional technological community is positively associated with the number of MedTech patent applications in the focal region. Moreover, the presence of MedTech inventors in the principal component of the regional technological community is positively associated with the number of MedTech patent applications in the region. However, local connections are not enough to promote innovation. In fact, the results show that intense cross-regional linkages of MedTech inventors increase the number of MedTech patent applications in the region. Thus, it is not only important that an inventor be well connected within her region, but also that she be exposed to external knowledge in order to increase her possibility of achieving high performance in MedTech within that region. Finally, I want also to understand how MedTech is open to other technological domains. I find that the average degree centrality and cross-regional linkages of academic inventors and inventors specialized in technologies complementary to MedTech affect regional innovation outcomes. The second work (chapter 3) is developed in collaboration with Dominique Foray. I aim to study the impact of external technologies on the MedTech sector. I start analyzing the literature of knowledge spillovers, and I do a comprehensive review of the extant measures of knowledge spillovers. I argue that the classical measures based on patent backward citations should be carefully interpreted. The reason as to why this type of index needs a prudent interpretation is linked to the characteristics of different technologies in terms of the speed at which other sectors are capable of understanding, absorbing and using them. Therefore, I propose a new formulation of the classical measure of backward citations. The third work (chapter 4) is developed in collaboration with Fabiana Visentin. I aim to understand the effects of the MedTech regulation that entered into force in Europe in 1993. I argue that the regulation has two effects. The first effect is related to the level of radicalness in the innovation. I claim that after the introduction of the regulation, and consequently with the tightening of the requirements to fulfill, firms became more careful and less motivated to propose radical innovations. At the same time, standardization of the requirements over the European countries gives to firms the possibility of widening their market.

EPFL2016