Giovanna Jona-Lasinio

Bayesian hidden Markov modelling using circular-linear general projected normal distribution

We introduce a multivariate hidden Markov model to jointly cluster time-series observations with different support, that is, circular and linear. Relying on the general projected normal distribution, our approach allows for bimodal and/or skewed cluster-specific distributions for the circular variable. Furthermore, we relax the independence assumption between the circular and linear components observed at the same time. Such an assumption is generally used to alleviate the computational burden involved in the parameter estimation step, but it is hard to justify in empirical applications. We carry out a simulation study using different data-generation schemes to investigate model behavior, focusing on well recovering the hidden structure. Finally, the model is used to fit a real data example on a bivariate time series of wind speed and direction. Copyright

Including spatial contiguity information in the analysis of multispecific patterns

In this paper we analyse an ordination method in which spatial contiguity information is included. The original method, proposed by Ver Hoef and Glenn-Lewin (1989), is extended, using graphs to describe spatial contiguity for several sampling schemes where locations are given. Extensions for two dimensional grids and for the case of discrete measures of abundance are analysed. Applications to simulated transect frequency data and real data collected on a regular grid are reported.

Exploring Multivariate Spatial Data: Line Transect Data

In this paper we describe an exploratory technique based on the diagonalization of cross-variogram matrices. Our aim is to describe the behavior of a multivariate set of spatial data in a dimensionally reduced space in such a way that the information on the spatial variation is preserved. Furthermore we propose a definition for the range of «variograms» in the multivariate case. Simulation studies and an application to botanical data collected on line transects are reported.

Space-time monitoring of coastal pollution in the Gulf of Gaeta, Italy, using δ 15 N values of Ulva lactuca , landscape hydromorphology, and Bayesian Kriging modelling

We investigated the space-time dynamics of N pollution in a Mediterranean gulf (Gulf of Gaeta) by means of δ15N variation in seaweed fronds (Ulva lactuca) previously collected from an unpolluted habitat. We used a comprehensive deployment grid that enabled the generation of isotopic seascapes (isoseascapes) describing the topography of N pollution in coastal waters and identifying N input hotspots and their pathways of dispersion at sea. The δ15N values of U. lactuca increased during 48h of exposure to the gulf waters, indicating anthropogenic N inputs from wastewater-derived sources. Comparison of the isoseascapes between two years differing in terms of rainfall identified coastal and offshore areas that were vulnerable to freshwater-transported nutrients, consistent with terrestrial hydromorphology and sea surface-water circulation. Isoseacapes were robust enough to reduce deployment effort, representing a powerful tool for monitoring and management strategies and useful for Environmental Protection Agencies, the main target audience of applied ecological research.

Exploring Multivariate Spatial Data: an Application to Election Data

In this paper we report a brief account of an application to spatial data, observed on an irregular grid, of an exploratory technique based on the diagonalization of cross-variogram matrices. Our aim is to describe the behavior of a multivariate set of spatial data in a dimensionally reduced space in such a way that the information on the spatial variation is preserved. We adapt an exploratory technique built for the analysis of quantitative data to frequency data. We give special attention to the choice of a distance measure that well describes the type of “connection” between sites we consider for the analysis of this specific situation. The application aims to the characterization of districts in the city of Rome according to the electoral behavior of their inhabitants, special attention is given to the increasing phenomenon of abstentions.