Estimating the causal effect of a time-varying treatment on time-to-event using structural nested failure time models

In this paper we review an approach to estimating the causal effect of a time-varying treatment on time to some event of interest. This approach is designed for the situation where the treatment may have been repeatedly adapted to patient characteristics, which themselves may also be time-dependent. In this situation the effect of the treatment cannot simply be estimated by conditioning on the patient characteristics, as these may themselves be indicators of the treatment effect. This so-called time-dependent confounding is typical in observational studies. We discuss a new class of failure time models, structural nested failure time models, which can be used to estimate the causal effect of a time-varying treatment, and present methods for estimating and testing the parameters of these models

Groundwater dependence and drought within the southern African development community

A groundwater situation analysis of the SADC region has been undertaken as part of the World Bank GEF Programme as a basis for ensuring equitable use of groundwater resources, particularly during periods of drought, both for human needs and for sustaining ecosystems. Much of the groundwater in the region occurs in weathered crystalline rocks suitable for dispersed supply to rural communities, although there are several aquifers capable of sustaining urban demand that contribute to the supply of several major cities and towns. A number of SADC Member States, such as Botswana, Namibia and South Africa, are very dependent on groundwater, whereas the Democratic Republic of Congo is least dependent. Groundwater dependence and groundwater demand, together providing an indication of drought vulnerability, have been assessed from the availability and coverage of groundwater data, but it is very apparent that reliable and comprehensive groundwater data are major deficiencies throughout the SADC region. Few attempts have thus been made to calculate renewable groundwater resource volumes or develop optimum use of groundwater, despite the fact that susceptibility of many Member States to drought requires them to consider mitigation strategies to lessen the hardships imposed largely on their rural population. Such strategy requires long-term intervention and not short-term emergency responses, a process that is directly related to availability of comprehensive groundwater datasets. Considerable effort in groundwater assessment and monitoring and the accumulation, evaluation and dissemination of essential datasets will thus be required to maintain population livelihoods in future years when water supply is projected to be in deficit in over half of the SADC Member States

Reducing Prawn-trawl Bycatch in Australia: An Overview and an Example from Queensland

Prawn trawling occurs in most states of Australia in tropical, subtropical, and temperate waters. Bycatch occurs to some degree in all Australian trawl fisheries, and there is pressure to reduce the levels of trawl fishery bycatch. This paper gives a brief overview of the bycatch issues and technological solutions that have been evaluated or adopted in Australian prawn-trawl fi sheries. Turtle excluder devices (TED’s) and bycatch reduction devices (BRD’s) are the principal solutions to bycatch in Australian prawn-trawl fisheries. This paper focuses on a major prawn-trawl fishery of northeastern Australia, and the results of commercial use of TED’s and BRD’s in the Queensland east coast trawl fishery are presented. New industry designs are described, and the status of TED and BRD adoption and regulation is summarized. The implementation of technological solutions to reduce fishery bycatch is assumed generally to assist prawn-trawl fisheries within Australia in achieving legislative requirements for minimal environmental impact and ecological sustainable development

Computational Topology Techniques for Characterizing Time-Series Data

Topological data analysis (TDA), while abstract, allows a characterization of time-series data obtained from nonlinear and complex dynamical systems. Though it is surprising that such an abstract measure of structure - counting pieces and holes - could be useful for real-world data, TDA lets us compare different systems, and even do membership testing or change-point detection. However, TDA is computationally expensive and involves a number of free parameters. This complexity can be obviated by coarse-graining, using a construct called the witness complex. The parametric dependence gives rise to the concept of persistent homology: how shape changes with scale. Its results allow us to distinguish time-series data from different systems - e.g., the same note played on different musical instruments.Comment: 12 pages, 6 Figures, 1 Table, The Sixteenth International Symposium on Intelligent Data Analysis (IDA 2017

Nested Markov Properties for Acyclic Directed Mixed Graphs

Directed acyclic graph (DAG) models may be characterized in at least four different ways: via a factorization, the d-separation criterion, the moralization criterion, and the local Markov property. As pointed out by Robins (1986, 1999), Verma and Pearl (1990), and Tian and Pearl (2002b), marginals of DAG models also imply equality constraints that are not conditional independences. The well-known `Verma constraint' is an example. Constraints of this type were used for testing edges (Shpitser et al., 2009), and an efficient marginalization scheme via variable elimination (Shpitser et al., 2011). We show that equality constraints like the `Verma constraint' can be viewed as conditional independences in kernel objects obtained from joint distributions via a fixing operation that generalizes conditioning and marginalization. We use these constraints to define, via Markov properties and a factorization, a graphical model associated with acyclic directed mixed graphs (ADMGs). We show that marginal distributions of DAG models lie in this model, prove that a characterization of these constraints given in (Tian and Pearl, 2002b) gives an alternative definition of the model, and finally show that the fixing operation we used to define the model can be used to give a particularly simple characterization of identifiable causal effects in hidden variable graphical causal models.Comment: 67 pages (not including appendix and references), 8 figure

Supersonic wings with significant leading-edge thrust at cruise

Experimental/theoretical correlations are presented which show that significant levels of leading edge thrust are possible at supersonic speeds for certain planforms which match the theoretical thrust distribution potential with the supporting airfoil geometry. The analytical process employed spanwise distribution of both it and/or that component of full theoretical thrust which acts as vortex lift. Significantly improved aerodynamic performance in the moderate supersonic speed regime is indicated

The SADC Groundwater Data and Information Archive, Knowledge Sharing and Co-operation Project. Final report

The Southern African Development Community (SADC) Groundwater Data and Information Archive, Knowledge Sharing and Co-operation Project, funded by the German Development Cooperation (GIZ) and Department for International Development, UK (DFID), was initiated in September 2009 to identify, catalogue and subsequently promote access to the large collection of reports held in the UK by the British Geological Survey (BGS). The work has focused on a wealth of unpublished so-called “grey” data and information which describes groundwater occurrence and development in Southern Africa and was gathered by the BGS over its many decades of involvement in the region. The project has four main aims: To catalogue and describe the "grey data" documents on SADC groundwater held by the BGS within a digital metadatabase. To identify a sub-set of scanned documents to be made freely available to groundwater practitioners and managers in the SADC region by electronic distribution. To link the metadatabase and digital sub-set of documents via a web portal hosted by the BGS, to enable download of documents by SADC groundwater workers. To strengthen links between BGS hydrogeologists with counterparts in SADC, and provide an example of groundwater data sharing which could be emulated by other European Geological Surveys with substantial data holdings on SADC groundwater. The project has successfully met these aims. The assessment of BGS archived material produced an electronic meta-database describing 1735 items held in hard copy. Of these, 1041 have been scanned digitally to searchable Portable Document Format (PDF) format. A subset of 655 PDFs including partial documents related to groundwater development from the colonial and post independence period as well as BGS internal project reports and reports approved for web dissemination by host countries are now available to download (free of charge) at http://www.SADCgroundwaterarchive.com . Initial results indicate a good deal of interest both from within SADC and elsewhere, accessed by directly addressing the website and via a search engine such as Google. The information presented has already been used by in-region projects such as the SADC Hydrogeological Mapping project and the Malawi Water Assessment Project. This is essentially a pilot project providing an example of how Web delivery of the archive is an important step forward for the well-being of the SADC region. It permits access to documents few even new existed and will, it is hoped, provide a valuable dataset that should inhibit the temptation to waste scarce resources by ‘re-inventing the wheel’

Betti number signatures of homogeneous Poisson point processes

The Betti numbers are fundamental topological quantities that describe the k-dimensional connectivity of an object: B_0 is the number of connected components and B_k effectively counts the number of k-dimensional holes. Although they are appealing natural descriptors of shape, the higher-order Betti numbers are more difficult to compute than other measures and so have not previously been studied per se in the context of stochastic geometry or statistical physics. As a mathematically tractable model, we consider the expected Betti numbers per unit volume of Poisson-centred spheres with radius alpha. We present results from simulations and derive analytic expressions for the low intensity, small radius limits of Betti numbers in one, two, and three dimensions. The algorithms and analysis depend on alpha-shapes, a construction from computational geometry that deserves to be more widely known in the physics community.Comment: Submitted to PRE. 11 pages, 10 figure