1,606 research outputs found

    The Impacts of the Americans with Disabilities Act on the Entry and Exit of Retail Firms

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    Congress enacted The Americans with Disabilities Act of 1990 over the protests of small business advocates who claimed that the ADA would trigger a wave of bankruptcies. Although the profitability of firms may suffer from the costs of ADA compliance, no systematic evidence is available on the ADA’s effect on firms. This paper seeks to determine if the ADA had a measurable impact on the number of retail firms, the entry of new firms, and the exit of existing firms in each market. I first develop a theoretical model of the response of industry dynamics to increases in costs (an extension of Klepper’s (1996) model). I show that increases in marginal and fixed costs may have interesting and non-obvious effects on entry and exit. The restrictions the model places on observed changes in entry and exit allow inference about which components of the ADA raised marginal cost and which raised fixed costs. The same model could easily be adapted to examine the impacts of other forms of cost-increasing regulation or exogenous process innovation on industry dynamics. The data used in the study are counts of business establishments currently operating by county and type of business. While standard count models can be used to investigate changes in the number of firms after the ADA, investigating the ADA’s impacts on entry and exit is more challenging. Backing out the entry and exit rates from the establishment count data is a major econometric contribution of the paper. Applying techniques from queuing theory, we develop the maximum likelihood estimator for a generalized Poisson queuing system based on the available count data. The model incorporates unobserved heterogeneity in and correlation between the entry and exit rates. Identification of the entry and exit rates is secured through the assumption that entry and exit are Poisson stochastic processes, conditional on time-varying covariates and correlated, gamma-distributed mixing terms (i.e., random effects that relax the Markovian assumptions in the model). Although we use techniques drawn from the existing queuing theory literature, the likelihood for the count data is non-trivial to derive and we have not seen the likelihood for this model presented elsewhere. We develop this model here out of necessity, due to the particular limitations of the available data; however, there are many other potential applications for the econometric model. The empirical results imply that the ADA indeed decreased the number of retail firms. There were fewer retail firms after the ADA was passed, and the drop was larger in states in which the ADA was more of a legal innovation, and in states that had more disabled people, more ADA-related lawsuits, and more ADA-related labor complaints. The same conclusions hold when baseline trends for larger establishments (those least vulnerable to the costs imposed by the ADA) are differenced out. Regarding entry and exit, there is also evidence that employment and access discrimination suits imposed real costs on retail stores, encouraging exit. However, the exit of incumbents was partially offset by new entrants, which may imply that stores less able to adapt to the new requirements made room for the entry of stores better able to adapt. So, while the prediction by the pessimists that the ADA would cause firms to fail may be correct, the decline in the number of firms was partially offset by new entry.Americans with disabilities act, industry dynamics, social regulation, queuing theory, heterogeneous Poisson stochastic process

    Birth/birth-death processes and their computable transition probabilities with biological applications

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    Birth-death processes track the size of a univariate population, but many biological systems involve interaction between populations, necessitating models for two or more populations simultaneously. A lack of efficient methods for evaluating finite-time transition probabilities of bivariate processes, however, has restricted statistical inference in these models. Researchers rely on computationally expensive methods such as matrix exponentiation or Monte Carlo approximation, restricting likelihood-based inference to small systems, or indirect methods such as approximate Bayesian computation. In this paper, we introduce the birth(death)/birth-death process, a tractable bivariate extension of the birth-death process. We develop an efficient and robust algorithm to calculate the transition probabilities of birth(death)/birth-death processes using a continued fraction representation of their Laplace transforms. Next, we identify several exemplary models arising in molecular epidemiology, macro-parasite evolution, and infectious disease modeling that fall within this class, and demonstrate advantages of our proposed method over existing approaches to inference in these models. Notably, the ubiquitous stochastic susceptible-infectious-removed (SIR) model falls within this class, and we emphasize that computable transition probabilities newly enable direct inference of parameters in the SIR model. We also propose a very fast method for approximating the transition probabilities under the SIR model via a novel branching process simplification, and compare it to the continued fraction representation method with application to the 17th century plague in Eyam. Although the two methods produce similar maximum a posteriori estimates, the branching process approximation fails to capture the correlation structure in the joint posterior distribution

    Artificial intelligence based decision support for trumpeter swan management

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    Department Head: Susan G. Stafford.2002 Spring.Includes bibliographical references (pages 108-114).The number of trumpeter swans (Cygnus buccinator) breeding in the Tri-State area where Montana, Idaho, and Wyoming come together has declined to just a few hundred pairs. However, these birds are part of the Rocky Mountain Population which additionally has over 3,500 birds breeding in Alberta, British Columbia, Northwest Territories, and Yukon Territory. To a large degree, these birds seem to have abandoned traditional migratory pathways in the flyway. Waterfowl managers have been interested in decision support tools that would help them explore simulated management scenarios in their quest towards reaching population recovery and the reestablishment of traditional migratory pathways. I have developed a decision support system to assist biologists with such management, especially related to wetland ecology. Decision support systems use a combination of models, analytical techniques, and information retrieval to help develop and evaluate appropriate alternatives. Swan management is a domain that is ecologically complex, and this complexity is compounded by spatial and temporal issues. The Distributed Environment Centered Agent Framework (DECAF) was successful at integrating communications among agents, integrating ecological knowledge, and simulating swan distributions through implementation of a queuing system. The work I have conducted indicates a need for determining what other factors might allow a deeper understanding of the effects of management actions on the flyway distribution of waterfowl. Knowing those would allow the more refined development of algorithms for effective decision support systems via collaboration by intelligent agents. Additional, specific conclusions and ideas for future research related both to waterfowl ecology and to the use of multiagent systems have been triggered by the validation work

    The Impacts of the Americans with Disabilities Act on the Entry and Exit of Retail Firms

    Get PDF
    Congress enacted The Americans with Disabilities Act of 1990 over the protests of small business advocates who claimed that the ADA would trigger a wave of bankruptcies. Although the profitability of firms may suffer from the costs of ADA compliance, no systematic evidence is available. This paper seeks to determine if the ADA had a measurable impact on both the entry of new firms and the failure rates (exit) of existing firms. The data used in the study are counts of business establishments currently operating by county and type of business. Backing out the entry and exit rates from the establishment count data is a major econometric contribution of the paper. The empirical results imply that the ADA indeed decreased the number of retail firms. There were fewer retail firms after the ADA was passed, and the drop was larger in states in which the ADA was more of a legal innovation, and in states that had more disabled people, more ADA-related lawsuits, and more ADA-related labor complaints. The same conclusions hold when baseline trends for larger establishments (those least vulnerable to the costs imposed by the ADA) are differenced out. There is also evidence that employment and access discrimination suits imposed real costs on retail stores, encouraging exit. However, the exit of incumbents was partially offset by new entrants, which may imply that stores less able to adapt to the new requirements made room for the entry of stores better able to adapt. So, while the prediction by the pessimists that the ADA would cause firms to fail may be correct, the decline in the number of firms was partially offset by new entry.entry, exit, count data models, queuing theory, ADA, industry dynamics

    Towards Runtime Verification of Programmable Switches

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    Is it possible to patch software bugs in P4 programs without human involvement? We show that this is partially possible in many cases due to advances in software testing and the structure of P4 programs. Our insight is that runtime verification can detect bugs, even those that are not detected at compile-time, with machine learning-guided fuzzing. This enables a more automated and real-time localization of bugs in P4 programs using software testing techniques like Tarantula. Once the bug in a P4 program is localized, the faulty code can be patched due to the programmable nature of P4. In addition, platform-dependent bugs can be detected. From P4_14 to P4_16 (latest version), our observation is that as the programmable blocks increase, the patchability of P4 programs increases accordingly. To this end, we design, develop, and evaluate P6 that (a) detects, (b) localizes, and (c) patches bugs in P4 programs with minimal human interaction. P6 tests P4 switch non-intrusively, i.e., requires no modification to the P4 program for detecting and localizing bugs. We used a P6 prototype to detect and patch seven existing bugs in eight publicly available P4 application programs deployed on two different switch platforms: behavioral model (bmv2) and Tofino. Our evaluation shows that P6 significantly outperforms bug detection baselines while generating fewer packets and patches bugs in P4 programs such as switch.p4 without triggering any regressions
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