On the application of loss functions for determining hazardous concentrations

The Hazardous Concentration to x% of an assemblage (HCx) of biological species is the environmental concentration which for a randomly selected species from the assemblage yields an x% probability of violating the species’ toxicological endpoint. Probabilistic methods for estimating the HCx appeal to the probabilistic concept of Species Sensitivity Distributions (SSDs) – a statistical proxy description of interspecies variation within the assemblage. A commonly used estimator class, derived by Aldenberg and Jaworska (2000; Ecotoxicol Environ Saf 46: 1-18), appealed to classical sampling theory, but also coincided with a Bayesian estimator. Two popular estimators from the class are the 50% and 95% (one-sided) underestimate of the HCx. However, whilst choice of x can have ecological significance, choice of confidence remains arbitrary. We reduce the problem to a Bayesian decision theoretic one; and show that their estimator class is equivalent to Bayes Rules under a class of (a-) symmetric linear loss functions, parameterised by the relative cost of over-estimation to under-estimation. A loss function in this sense measures the ‘cost’, which needn’t be monetary, of over- and under-estimation of the HCx estimator. Bayes rules are estimators which minimise expected loss with respect to the posterior SSD – updated with respect to the toxicity data. This potentially opens the way for high-stakes realism to be incorporated into risk assessments. We propose an alternative loss function known as Scaled LINear Exponential (LINEX) which is non-linearly asymmetric in a precautionary way, such that overestimation and underestimation are punished at an exponential and linear rate respectively. We use this loss function to derive an alternative class of HCx estimators

The evolution of the spatial pattern of white residential development and the housing market in Johannesburg

A thesis submitted to the Faculty of Arts, University of the Witwatersrand, Johannesburg for the degree of Doctor of Philosophy Johannesburg, 1974The problem of residential development and the price of residential property is investigated through an empirical study of the city of Johannesburg from its inception in 1886 to 1972. The analysis is temporal and spatial and is based upon municipal valuations and sales data of residential property. The analytic framework is developed from urban growth theory on the one hand, and the theory of the urban land market, on the other. The findings reveal that both residential development and residential property prices are dependent upon a wide variety of socially rooted and economically rooted determinants and that these act in concert to produce the dynamic spatial pattern found in the urban system

Species non-exchangeability in probabilistic ecotoxicological risk assessment

Current ecotoxicological risk assessment for chemical substances is based on the assumption that tolerances of all species in a specified ecological community are a priori exchangeable for each new substance. We demonstrate non-exchangeability by using a large database of tolerances to pesticides for fish species and extend the standard statistical model for species tolerances to allow for the presence of a single species which is considered non-exchangeable with others. We show how to estimate parameters and adjust decision rules that are used in ecotoxicological risk management. Effects of parameter uncertainty are explored and our model is compared with a previously published less tractable alternative. We conclude that the model and decision rules that we propose are pragmatic compromises between conflicting needs for more realistic modelling and for straightforwardly applicable decision rules

Species non-exchangeability for ecotoxicological risk assessment [poster spotlight]

In aquatic based chemical risk assessments, there is a wealth of statistical techniques for use in lower tier risk assessment. In particular, we focus on estimation of the hazardous concentration to x% of an ecological community (HCx); a concept based on the idea of Species Sensitivity Distribution (SSD). The SSD is typically assumed to act as a proxy distribution to model the inter-species variation in the biological assemblage. Over time, a number of criticisms have been made of the SSD concept, but we focus on one in particular – species non-exchangeability. The concept was first discussed within a semi-probabilistic setting by an opinion of the European Food Safety Authority (EFSA) Scientific Panel on Plant Production products and their Residues (EFSA Journal, 2005). We build on their findings to demonstrate, statistically, that the Rainbow trout (Oncorhynchus mykiss) is not exchangeable with other species. By this term, we mean that, a priori, before observing the toxicity value of the species, we do not believe it to be a realisation from the same distribution as the other species in the assemblage. In fact, the Rainbow trout is typically more sensitive than the average fish species across a wide range of substances. In addition, we briefly demonstrate how to exploit historical databases of toxicity data featuring the Rainbow trout to quantify this non-exchangeability in order to derive new estimators for the HCx

Introduction of Medical Emergency Teams in Australia and New Zealand: a multi-centre study

INTRODUCTION: Information about Medical Emergency Teams (METs) in Australia and New Zealand (ANZ) is limited to local studies and a cluster randomised controlled trial (the Medical Emergency Response and Intervention Trial [MERIT]). Thus, we sought to describe the timing of the introduction of METs into ANZ hospitals relative to relevant publications and to assess changes in the incidence and rate of intensive care unit (ICU) admissions due to a ward cardiac arrest (CA) and ICU readmissions. METHODS: We used the Australian and New Zealand Intensive Care Society database to obtain the study data. We related MET introduction to publications about adverse events and MET services. We compared the incidence and rate of readmissions and admitted CAs from wards before and after the introduction of an MET. Finally, we identified hospitals without an MET system which had contributed to the database for at least two years from 2002 to 2005 and measured the incidence of adverse events from the first year of contribution to the second. RESULTS: The MET status was known for 131 of the 172 (76.2%) hospitals that did not participate in the MERIT study. Among these hospitals, 110 (64.1%) had introduced an MET service by 2005. In the 79 hospitals in which the MET commencement date was known, 75% had introduced an MET by May 2002. Of the 110 hospitals in which an MET service was introduced, 24 (21.8%) contributed continuous data in the year before and after the known commencement date. In these hospitals, the mean incidence of CAs admitted to the ICU from the wards changed from 6.33 per year before to 5.04 per year in the year after the MET service began (difference of 1.29 per year, 95% confidence interval [CI] -0.09 to 2.67; P = 0.0244). The incidence of ICU readmissions and the mortality for both ICU-admitted CAs from wards and ICU readmissions did not change. Data were available to calculate the change in ICU admissions due to ward CAs for 16 of 62 (25.8%) hospitals without an MET system. In these hospitals, admissions to the ICU after a ward CA decreased from 5.0 per year in the first year of data contribution to 4.2 per year in the following year (difference of 0.8 per year, 95% CI -0.81 to 3.49; P = 0.3). CONCLUSION: Approximately 60% of hospitals in ANZ with an ICU report having an MET service. Most introduced the MET service early and in association with literature related to adverse events. Although available in only a quarter of hospitals, temporal trends suggest an overall decrease in the incidence of ward CAs admitted to the ICU in MET as well as non-MET hospitals

Poster Abstract:Deploying a 6LoWPAN, CoAP, low power, wireless sensor network

In order to integrate equipment from different vendors, wireless sensor networks need to become more standardized. Using IP as the basis of low power radio networks, together with application layer standards designed for this purpose is one way forward. This research focuses on implementing and deploying a system using Contiki, 6LoWPAN over an 868 MHz radio network, together with CoAP as a standard application layer protocol. A system was deployed in the Cairngorm mountains in Scotland as an environmental sensor network, measuring streams, temperature profiles in peat and periglacial features. It was found that RPL provided an effective routing algorithm, and that the use of UDP packets with CoAP proved to be an energy efficient application layer. This combination of technologies can be very effective in large area sensor networks

Britain’s highest bog: can we unlock its secrets?

The Glenfeshie Mòine Mhór (Great Moss) is Britain’s highest bog, the largest bog in the Cairngorm Mountains (Scotland) and a water source area for the River Spey. The area was managed primarily for sport hunting for about two centuries, but deer numbers have been heavily reduced in the last decade to allow regeneration of natural woodland and the return to more natural condition of all ecosystems including peatland. However, it may not be realistic to expect spontaneous improvement in peatland condition and ecosystem services provision in the harsh environment of the Mòine Mhór, which retains snow cover for more than half the year and differs floristically from lower-altitude bogs. To understand whether and where management intervention may be required, we need first to understand how the system works at scales ranging from microform to macrotope, and from sub-catchment to whole-system level. Multi-disciplinary condition and process studies (involving various collaborators) are in progress, with a current emphasis on streamflow generation and fluvial carbon loads. This presentation develops two sub-themes. First, ground survey and GIS analysis are used to address the questions: what are the special features of this bog; what is the nature and extent of degradation; and what are the implications for water delivered to the outflow streams? Secondly, a striking feature is the bare peat patches which were favourite resting places for deer on warm, dry summer days. The occurrence of seasonally extreme surface conditions seems a likely factor in preventing their recolonisation by bog plants now. Information about these conditions that cannot readily be accessed through direct observation, originating from temperature sensors and delivered at 60-minute intervals via a low power internet link, is explored in this context. Finally, we discuss aspects of the suitability of our investigation methods for remote and intermittently accessible field sites such as the Mòine Mhór

Long term effect of a medical emergency team on cardiac arrests in a teaching hospital

INTRODUCTION: It is unknown whether the reported short-term reduction in cardiac arrests associated with the introduction of the medical emergency team (MET) system can be sustained. METHOD: We conducted a prospective, controlled before-and-after examination of the effect of a MET system on the long-term incidence of cardiac arrests. We included consecutive patients admitted during three study periods: before the introduction of the MET; during the education phase preceding the implementation of the MET; and a period of four years from the implementation of the MET system. Cardiac arrests were identified from a log book of cardiac arrest calls and cross-referenced with case report forms and the intensive care unit admissions database. We measured the number of hospital admissions and MET reviews during each period, performed multivariate logistic regression analysis to identify predictors of mortality following cardiac arrest and studied the correlation between the rate of MET calls with the rate of cardiac arrests. RESULTS: Before the introduction of the MET system there were 66 cardiac arrests and 16,246 admissions (4.06 cardiac arrests per 1,000 admissions). During the education period, the incidence of cardiac arrests decreased to 2.45 per 1,000 admissions (odds ratio (OR) for cardiac arrest 0.60; 95% confidence interval (CI) 0.43–0.86; p = 0.004). After the implementation of the MET system, the incidence of cardiac arrests further decreased to 1.90 per 1,000 admissions (OR for cardiac arrest 0.47; 95% CI 0.35–0.62; p < 0.0001). There was an inverse correlation between the number of MET calls in each calendar year and the number of cardiac arrests for the same year (r(2 )= 0.84; p = 0.01), with 17 MET calls being associated with one less cardiac arrest. Male gender (OR 2.88; 95% CI 1.34–6.19) and an initial rhythm of either asystole (OR 7.58; 95% CI 3.15–18.25; p < 0.0001) or pulseless electrical activity (OR 4.09; 95% CI 1.59–10.51; p = 0.003) predicted an increased risk of death. CONCLUSION: Introduction of a MET system into a teaching hospital was associated with a sustained and progressive reduction in cardiac arrests over a four year period. Our findings show sustainability and suggest that, for every 17 MET calls, one cardiac arrest might be prevented