168 research outputs found
Geomedia: Networked Cities and the Future of Public Space
Written by Scott McQuir
Disentangling agglomeration and network externalities : a conceptual typology
Agglomeration and network externalities are fuzzy concepts. When different meanings are (un)intentionally juxtaposed in analyses of the agglomeration/network externalities-menagerie, researchers may reach inaccurate conclusions about how they interlock. Both externality types can be analytically combined, but only when one adopts a coherent approach to their conceptualization and operationalization, to which end we provide a combinatorial typology. We illustrate the typology by applying a state-of-the-art bipartite network projection detailing the presence of globalized producer services firms in cities in 2012. This leads to two one-mode graphs that can be validly interpreted as topological renderings of agglomeration and network externalities
Mixing time for uniform sampling of bipartite graphs with fixed degrees using the trade algorithm
Uniform sampling of bipartite graphs and hypergraphs with given degree
sequences is necessary for building null models to statistically evaluate their
topology. Because these graphs can be represented as binary matrices, the
problem is equivalent to uniformly sampling binary matrices with
fixed row and column sums. The trade algorithm, which includes both the
curveball and fastball implementations, is the state-of-the-art for performing
such sampling. Its mixing time is currently unknown, although is currently
used as a heuristic. In this paper we propose a new distribution-based approach
that not only provides an estimation of the mixing time, but also actually
returns a sample of matrices that are guaranteed (within a user-chosen error
tolerance) to be uniformly randomly sampled. In numerical experiments on
matrices that vary by size, fill, and row and column sum distributions, we find
that the upper bound on mixing time is at least , and that it increases as
a function of both and the fraction of cells containing a 1
The duality of networks and groups: Models to generate two-mode networks from one-mode networks
Focus theory describes how shared memberships, social statuses, beliefs, and
places can facilitate the formation of social ties, while two-mode projections
provide a method for transforming two-mode data on individuals' memberships in
groups into a one-mode network of their possible social ties. In this paper, I
explore the opposite process: how social ties can facilitate the formation of
groups, and how a two-mode network can be generated from a one-mode network.
Drawing on theories of team formation, club joining, and organization
recruitment, I propose three models that describe how such groups might emerge
from the relationships in a social network. I show that these models can be
used to generate two-mode networks that have characteristics commonly observed
in empirical two-mode social networks, and that they encode features of the
one-mode networks from which they were generated. I conclude by discussing
these models' limitations, and future directions for theory and methods
concerning group formation
Inferring social networks from observed groups
Collecting social network data directly from network members can be
challenging. One alternative involves inferring a social network from
individuals' memberships in observed groups, such as teams or clubs. Through a
series of simulations, I explore when we can expect such inferences to be
accurate. I find that an unobserved network can be inferred with high accuracy
under a range of circumstances. In particular, I find that social networks
inferred from observed groups are more accurate when (1) the unobserved network
has a small world structure, (2) the groups are generated by a shuffling or
agglomerative process, (3) a large number of groups are observed, and (4) the
observed groups' compositions are tightly coupled to the unobserved network's
structure. These findings offer guidance for researchers seeking to indirectly
measure a social network of interest through observations of groups
Stochastic Degree Sequence Model with Edge Constraints (SDSM-EC) for Backbone Extraction
It is common to use the projection of a bipartite network to measure a
unipartite network of interest. For example, scientific collaboration networks
are often measured using a co-authorship network, which is the projection of a
bipartite author-paper network. Caution is required when interpreting the edge
weights that appear in such projections. However, backbone models offer a
solution by providing a formal statistical method for evaluating when an edge
in a projection is statistically significantly strong. In this paper, we
propose an extension to the existing Stochastic Degree Sequence Model (SDSM)
that allows the null model to include edge constraints (EC) such as prohibited
edges. We demonstrate the new SDSM-EC in toy data and empirical data on young
children's' play interactions, illustrating how it correctly omits noisy edges
from the backbone
backbone: An R Package for extracting the backbone of bipartite projections
Bipartite projections are used in a wide range of network contexts including
politics (bill co-sponsorship), genetics (gene co-expression), economics
(executive board co-membership), and innovation (patent co-authorship).
However, because bipartite projections are always weighted graphs, which are
inherently challenging to analyze and visualize, it is often useful to examine
the 'backbone', an unweighted subgraph containing only the most significant
edges. In this paper, we introduce the R package backbone for extracting the
backbone of weighted bipartite projections, and use bill sponsorship data from
the 114th session of the United States Senate to demonstrate its functionality
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