128 research outputs found
Network Effects in Alternative Fuel Adoption: Empirical Analysis of the Market for Ethanol
This paper investigates the importance of network effects in the demand
for ethanol-compatible vehicles and the supply of ethanol fuel
retailers. An indirect network effect, or positive feedback loop, arises
in this context due to spatially-dependent complementarities in the
availability of ethanol fuel and the installed base of
ethanol-compatible vehicles. Marketers and social planners are
interested in whether these effects exist, and if so, how policy might
accelerate adoption of the ethanol fuel standard within a targeted
population. To measure these feedback effects, I develop an econometric
framework that considers the simultaneous determination of
ethanol-compatible vehicle demand and ethanol fuel supply in local
markets. The demand-side of the model considers the automobile purchase
decisions of consumers and fleet operators, and the supply-side model
considers the ethanol market entry decisions of competing fuel
retailers. I propose new estimators that address the endogeneity induced
by the co-determination of alternative fuel vehicle demand and
alternative fuel supply. I estimate the model using zip code level panel
data from six states over a six year period. I find the network effect
to be highly signifi cant, both statistically and economically. Under
typical market conditions, entry of an additional ethanol fuel retailer
leads to a 12% increase in consumer demand for ethanol-compatible
vehicles. The entry model estimates imply that a monopolist requires a
local installed base of at least 204 ethanol-compatible vehicles to be
pro table. As an application, I demonstrate how the model estimates can
inform the promotional strategy of a vehicle manufacturer.
Counter-factual simulations indicate that subsidizing fuel retailers to
offer ethanol can be an effective policy to indirectly increase
ethanol-compatible vehicle sales
Nonlinear cascades of surface oceanic geostrophic kinetic energy in the frequency domain
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111877/1/jpo_frequencycascades_2012.pd
On eddy viscosity, energy cascades, and the horizontal resolution of gridded satellite altimeter products
Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 43 (2013): 283–300, doi:10.1175/JPO-D-11-0240.1.Motivated by the recent interest in ocean energetics, the widespread use of horizontal eddy viscosity in models, and the promise of high horizontal resolution data from the planned wide-swath satellite altimeter, this paper explores the impacts of horizontal eddy viscosity and horizontal grid resolution on geostrophic turbulence, with a particular focus on spectral kinetic energy fluxes Π(K) computed in the isotropic wavenumber (K) domain. The paper utilizes idealized two-layer quasigeostrophic (QG) models, realistic high-resolution ocean general circulation models, and present-generation gridded satellite altimeter data. Adding horizontal eddy viscosity to the QG model results in a forward cascade at smaller scales, in apparent agreement with results from present-generation altimetry. Eddy viscosity is taken to roughly represent coupling of mesoscale eddies to internal waves or to submesoscale eddies. Filtering the output of either the QG or realistic models before computing Π(K) also greatly increases the forward cascade. Such filtering mimics the smoothing inherent in the construction of present-generation gridded altimeter data. It is therefore difficult to say whether the forward cascades seen in present-generation altimeter data are due to real physics (represented here by eddy viscosity) or to insufficient horizontal resolution. The inverse cascade at larger scales remains in the models even after filtering, suggesting that its existence in the models and in altimeter data is robust. However, the magnitude of the inverse cascade is affected by filtering, suggesting that the wide-swath altimeter will allow a more accurate determination of the inverse cascade at larger scales as well as providing important constraints on smaller-scale dynamics.BKA received support from Office of Naval
Research Grant N00014-11-1-0487, National Science
Foundation (NSF) Grants OCE-0924481 and OCE-
09607820, and University of Michigan startup funds.
KLP acknowledges support from Woods Hole Oceanographic
Institution bridge support funds. RBS acknowledges
support from NSF grants OCE-0960834 and
OCE-0851457, a contract with the National Oceanography
Centre, Southampton, and a NASA subcontract
to Boston University. JFS and JGR were supported by
the projects ‘‘Global and remote littoral forcing in
global ocean models’’ and ‘‘Agesotrophic vorticity dynamics
of the ocean,’’ respectively, both sponsored by
the Office of Naval Research under program element 601153N.2013-08-0
Effects of stencil width on surface ocean geostrophic velocity and vorticity estimation from gridded satellite altimeter data
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90421/1/2011JC007367.pd
Social Ties and User Generated Content: Evidence from an Online Social Network
We use variation in wind speeds at surfing locations in Switzerland as
exogenous shifters of users' propensity to post content about their
surfing activity onto an online social network. We exploit this
variation to test whether users' social ties on the network have a
causal effect on their content generation, and whether conent generation
in turn has a similar causal effect on the users' abilty to form social
ties. Economically significant causal effects of this kind can produce
positive feedback that generate multiplier e¤ects to
interventions that subsidize tie formation. We argue these interventions
can therefore be the basis of a strategy by the rm to indirectly
faciliate content generation on the site. The exogenous variation
provided by wind speeds enable us to measure this feedback empirically
and to assess the return on investment from such policies. We use a
detailed dataset from an online social network that comprises the
complete details of social tie formation and content generation on the
site. The richness of he data enable us to control for several spurious
confounds that have typically plagued empirical analysis of social
interactions. Our results show evidence for significant positive
feedback in user generated content. We discuss the implications of the
estimates for the management of the content and the growth of the network
Toward an internal gravity wave spectrum in global ocean models
Office of Naval Research, National Science FoundationPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111824/1/grl_internalwavespectrum_2015.pd
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Effects of stencil width on surface ocean geostrophic velocity and vorticity estimation from gridded satellite altimeter data
This paper examines the effect of “stencil width” on surface ocean geostrophic velocity and vorticity estimated from differentiating gridded satellite altimeter sea surface height products. In oceanographic applications, the value of the first derivative at a central grid point is generally obtained by differencing the sea surface heights at adjacent grid points. This is called a “three-point stencil centered difference”. Here the stencil width is increased from three to five, seven, and nine points, using well-known formulae from the numerical analysis literature. The discrepancies between velocities computed with successive stencils decreases with increasing stencil width, suggesting that wide stencil results are more reliable. Significant speed-dependent biases (up to 10–20%) are found between results computed from three-point stencils versus those computed from wider stencils. The geostrophic velocity, and the variance of geostrophic velocity, are underestimated with thin stencils. Similar results are seen in geostrophic velocities computed from high-resolution model output. In contrast to the case when three-point stencils are used, wider stencils yield estimates of the anisotropy of velocity variance that are insensitive to the differences in grid spacing between two widely used altimeter products. Three-point stencils yield incorrect anisotropies on the 1/4° anisotropic AVISO grid; we recommend the use of 7-point stencils. Despite the demonstrated inadequacies of the three-point stencils, the conclusions of earlier studies based on them, that the zonally averaged midlatitude eddy kinetic energy field is nearly isotropic, are found to pertain also with wider stencils. Finally, the paper also examines the strengths and limitations of applying noise-suppressing differentiators, versus classic centered differences, to altimeter data
Social Ties and User Generated Content: Evidence from an Online Social Network
We use variation in wind speeds at surfing locations in Switzerland as
exogenous shifters of users' propensity to post content about their
surfing activity onto an online social network. We exploit this
variation to test whether users' social ties on the network have a
causal effect on their content generation, and whether conent generation
in turn has a similar causal effect on the users' abilty to form social
ties. Economically significant causal effects of this kind can produce
positive feedback that generate multiplier e¤ects to
interventions that subsidize tie formation. We argue these interventions
can therefore be the basis of a strategy by the rm to indirectly
faciliate content generation on the site. The exogenous variation
provided by wind speeds enable us to measure this feedback empirically
and to assess the return on investment from such policies. We use a
detailed dataset from an online social network that comprises the
complete details of social tie formation and content generation on the
site. The richness of he data enable us to control for several spurious
confounds that have typically plagued empirical analysis of social
interactions. Our results show evidence for significant positive
feedback in user generated content. We discuss the implications of the
estimates for the management of the content and the growth of the network
Pan-Cancer Analysis of lncRNA Regulation Supports Their Targeting of Cancer Genes in Each Tumor Context
Long noncoding RNAs (lncRNAs) are commonly dys-regulated in tumors, but only a handful are known toplay pathophysiological roles in cancer. We inferredlncRNAs that dysregulate cancer pathways, onco-genes, and tumor suppressors (cancer genes) bymodeling their effects on the activity of transcriptionfactors, RNA-binding proteins, and microRNAs in5,185 TCGA tumors and 1,019 ENCODE assays.Our predictions included hundreds of candidateonco- and tumor-suppressor lncRNAs (cancerlncRNAs) whose somatic alterations account for thedysregulation of dozens of cancer genes and path-ways in each of 14 tumor contexts. To demonstrateproof of concept, we showed that perturbations tar-geting OIP5-AS1 (an inferred tumor suppressor) andTUG1 and WT1-AS (inferred onco-lncRNAs) dysre-gulated cancer genes and altered proliferation ofbreast and gynecologic cancer cells. Our analysis in-dicates that, although most lncRNAs are dysregu-lated in a tumor-specific manner, some, includingOIP5-AS1, TUG1, NEAT1, MEG3, and TSIX, synergis-tically dysregulate cancer pathways in multiple tumorcontexts
Genomic, Pathway Network, and Immunologic Features Distinguishing Squamous Carcinomas
This integrated, multiplatform PanCancer Atlas study co-mapped and identified distinguishing
molecular features of squamous cell carcinomas (SCCs) from five sites associated with smokin
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