Business Administration - Research Publications
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ItemMitigating spatial confounding by explicitly correlating Gaussian random fields(Wiley, 2022-08-01)
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ItemVariational inference and sparsity in high-dimensional deep Gaussian mixture models(Springer Science and Business Media LLC, 2022-10-01)Abstract Gaussian mixture models are a popular tool for model-based clustering, and mixtures of factor analyzers are Gaussian mixture models having parsimonious factor covariance structure for mixture components. There are several recent extensions of mixture of factor analyzers to deep mixtures, where the Gaussian model for the latent factors is replaced by a mixture of factor analyzers. This construction can be iterated to obtain a model with many layers. These deep models are challenging to fit, and we consider Bayesian inference using sparsity priors to further regularize the estimation. A scalable natural gradient variational inference algorithm is developed for fitting the model, and we suggest computationally efficient approaches to the architecture choice using overfitted mixtures where unnecessary components drop out in the estimation. In a number of simulated and two real examples, we demonstrate the versatility of our approach for high-dimensional problems, and demonstrate that the use of sparsity inducing priors can be helpful for obtaining improved clustering results.
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ItemA non-stationary model for spatially dependent circular response data based on wrapped Gaussian processes(Springer Science and Business Media LLC, 2022-10-01)Abstract Circular data can be found across many areas of science, for instance meteorology (e.g., wind directions), ecology (e.g., animal movement directions), or medicine (e.g., seasonality in disease onset). The special nature of these data means that conventional methods for non-periodic data are no longer valid. In this paper, we consider wrapped Gaussian processes and introduce a spatial model for circular data that allow for non-stationarity in the mean and the covariance structure of Gaussian random fields. We use the empirical equivalence between Gaussian random fields and Gaussian Markov random fields which allows us to considerably reduce computational complexity by exploiting the sparseness of the precision matrix of the associated Gaussian Markov random field. Furthermore, we develop tunable priors, inspired by the penalized complexity prior framework, that shrink the model toward a less flexible base model with stationary mean and covariance function. Posterior estimation is done via Markov chain Monte Carlo simulation. The performance of the model is evaluated in a simulation study. Finally, the model is applied to analyzing wind directions in Germany.
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ItemWealth in People and Places: Understanding Transnational Gift Obligations(OXFORD UNIV PRESS INC, 2022-06-30)Abstract In some cultures, migrants bear an obligation to bring gifts from the foreign country for their relations when returning to their homeland. Why, and to what end? We examine the reasons for these transnational gift obligations in a multisite study of Ghanaian migrants in the United States and Australia, as well as people in Ghana with migrant relations living overseas. We adopt a wealth-centered perspective that problematizes the underexplored mutual impact of migrants and their gifts on social hierarchies within societies and transnational spatial hierarchies between societies. We show how the concepts of wealth in people and wealth in place connect with local gift economies to explain transnational gifting obligations. Specifically, informants use transnational gifts that embody wealth in place to acknowledge “being wealth” to people and to acquire wealth in others. We highlight the wealth in things that are exchanged as gift objects and the wealth in people who are exchanged as gift subjects between here and there. Our findings implicate a “glocal” gift economy that results from the global flows of things and people as gifts within transnational places of differing statuses. We discuss how this glocal gift economy (re)produces transnational spatial hierarchies and local (national) status hierarchies.
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ItemThe rise of 'smart' solutions in Africa: a review of the socio-environmental cost of the transportation and employment benefits of ride-hailing technology in Ghana.(Springer Science and Business Media LLC, 2022)Governments in Africa are licensing major global ride-hailing firms to launch operations in the continent. This is often presented as a refreshing development for the continent to leverage technology to address its twin problems of inefficient urban transport and rising youth unemployment. Interviews with ride-hailing adopters (drivers, riders, and car owners) and researchers in Ghana suggest, however, that whereas the technology is driving up the standards of road transport experience, the benefits are accessible to a select few (largely, the younger, highly educated and relatively high income-earning class). The lopsided power relations underlying the ride-hailing industry have also meant that the economic opportunities it avails disproportionately benefit a few powerful players (e.g. ride-hailing firms and car owners) while stimulating 'turf wars' among online and traditional taxi drivers; deepening existing gender inequalities in access to income-earning opportunities in the commercial passenger transport sector; encouraging unhealthy driving practices, shifts from shared public transport, and inundation of the roads with more private cars. While it will be imprecise to say that the private gains of ride-hailing outstrip the public costs and, therefore, the technology is detrimental to Ghana's development, the considered evidence raises the need for sustained scrutiny of the hailing of technological interventions as though they are the magic bullets for socio-economic transformation in Africa. Overall, the paper argues that dismantling the power structures underlying Africa's urban challenges will require more than splashing 'smart' apps and other tech wizardries around. Indeed, the lessons from Ghana's ride-hailing industry suggest that such exclusively technical solutions could easily take root and pattern after existing strictures of unjust power structures in ways that could exacerbate the social and environmental problems they are supposed to address.
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ItemIs age at menopause decreasing? - The consequences of not completing the generational cohort.(Springer Science and Business Media LLC, 2022-07-11)BACKGROUND: Due to contradictory results in current research, whether age at menopause is increasing or decreasing in Western countries remains an open question, yet worth studying as later ages at menopause are likely to be related to an increased risk of breast cancer. Using data from breast cancer screening programs to study the temporal trend of age at menopause is difficult since especially younger women in the same generational cohort have often not yet reached menopause. Deleting these younger women in a breast cancer risk analyses may bias the results. The aim of this study is therefore to recover missing menopause ages as a covariate by comparing methods for handling missing data. Additionally, the study makes a contribution to understanding the evolution of age at menopause for several generations born in Portugal between 1920 and 1970. METHODS: Data from a breast cancer screening program in Portugal including 278,282 women aged 45-69 and collected between 1990 and 2010 are used to compare two approaches of imputing age at menopause: (i) a multiple imputation methodology based on a truncated distribution but ignoring the mechanism of missingness; (ii) a copula-based multiple imputation method that simultaneously handles the age at menopause and the missing mechanism. The linear predictors considered in both cases have a semiparametric additive structure accommodating linear and non-linear effects defined via splines or Markov random fields smoothers in the case of spatial variables. RESULTS: Both imputation methods unveiled an increasing trend of age at menopause when viewed as a function of the birth year for the youngest generation. This trend is hidden if we model only women with an observed age at menopause. CONCLUSION: When studying age at menopause, missing ages must be recovered with an adequate procedure for incomplete data. Imputing these missing ages avoids excluding the younger generation cohort of the screening program in breast cancer risk analyses and hence reduces the bias stemming from this exclusion. In addition, imputing the not yet observed ages of menopause for mostly younger women is also crucial when studying the time trend of age at menopause otherwise the analysis will be biased.
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ItemUsing Background Knowledge from Preceding Studies for Building a Random Forest Prediction Model: A Plasmode Simulation Study.(MDPI AG, 2022-06-20)There is an increasing interest in machine learning (ML) algorithms for predicting patient outcomes, as these methods are designed to automatically discover complex data patterns. For example, the random forest (RF) algorithm is designed to identify relevant predictor variables out of a large set of candidates. In addition, researchers may also use external information for variable selection to improve model interpretability and variable selection accuracy, thereby prediction quality. However, it is unclear to which extent, if at all, RF and ML methods may benefit from external information. In this paper, we examine the usefulness of external information from prior variable selection studies that used traditional statistical modeling approaches such as the Lasso, or suboptimal methods such as univariate selection. We conducted a plasmode simulation study based on subsampling a data set from a pharmacoepidemiologic study with nearly 200,000 individuals, two binary outcomes and 1152 candidate predictor (mainly sparse binary) variables. When the scope of candidate predictors was reduced based on external knowledge RF models achieved better calibration, that is, better agreement of predictions and observed outcome rates. However, prediction quality measured by cross-entropy, AUROC or the Brier score did not improve. We recommend appraising the methodological quality of studies that serve as an external information source for future prediction model development.
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ItemOnline community's recognition and continued participation in idea competitions(Wiley, 2022-07-20)This study examines the effect of online community's recognition on continued participation in idea competitions, and how personal winning record moderates such an influence. We reason that the motivating role of community recognition might either be reinforced or substituted by personal winning record, depending upon whether relational motives (psychological and social bonding) or individualistic motives (personal benefits, such as status and career enhancement) are the primary behavior driver. Through an event history analysis of data obtained from a platform of creative design contests, we find that although community recognition exerts a positive effect on the rate of continued participation for designers who are yet to win any competitions, this effect increasingly turns negative for designers who have won. Such findings indicate that the motivating role of community recognition might be substituted instead of reinforced by personal winning record, lending support to the individualistic view while rejecting the relational view. Although virtual social spaces represent an important means for modern competition platforms to attract and motivate participants, our study informs practitioners about online community's limitation in retaining “star” participants.
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ItemNo Preview Availablebamlss: A Lego Toolbox for Flexible Bayesian Regression (and Beyond)(JOURNAL STATISTICAL SOFTWARE, 2021-11-01)