This paper examines the usability of the hypertext navigation facilities provided by World Wide Web client applications. A notation is defined to represent the user's navigational acts and the resultant system states. The notation is used to report potential, or 'theoretical,' problems in the models of navigation supported by three web client applications. A usability study confirms that these problems emerge in actual use, and demonstrates that incorrect user models of the clients' facilities are common. A usability analysis identifies inadequacies in the clients' interfaces.Motivated by the analysis of usability problems, we propose extensions to the design of WWW client applications. These proposals are demonstrated by our system WebNetwhich uses dynamic graphical overview diagrams to extend the navigational facilities of conventional World Wide Web client applications. Related work on graphical overview diagrams for web navigation is reviewed.
The history of liquid and atmospheric discharges from the Sellafield nuclear complex in Cumbria, UK over the period from 1951 (the start of operations at the site) to 1992 has been reviewed. The chronology of liquid effluent discharges is essentially unchanged from previously published figures, but the chronology of aerial effluent discharges has been very substantially amended, reflecting the results of experimental work to assess the efficiency of aerial effluent sampling systems on the site, the analysis of previously unreported data on plant based measurements of aerial effluents in the 1950s, and a number of other factors. The amended discharge chronologies have been validated by reference to the extensive environmental monitoring data which are available for the Sellafield site, comparing environmental concentrations predicted from the discharge chronology and environmental models with those actually measured. The revised discharge chronologies together with the model account for all the major features of spatial and temporal variations in environmental concentrations of radionuclides in the vicinity of the site, but overestimate the body content of plutonium in local residents compared to the limited number of available measurements. The revised discharge chronologies do not lead to any major changes in the assessed lifetime doses to local residents and do not alter the conclusion that doses from discharges appear to be far too small to account for the excess of childhood leukaemia in the local village of Seascale.
A framework for protection of the environment is likely to require a methodology for assessing dose rates arising from naturally occurring radionuclides. This paper addresses this issue for European aquatic environments through a process of (a) data collation, mainly with respect to levels of radioactivity in water sediments and aquatic flora and fauna, (b) the use of suitable distribution coefficients, concentration factors and global data where data gaps are present and (c) the utilisation of a reference organism approach whereby a finite number of suitable geometries are selected to allow dose per unit concentration factors to be derived and subsequent absorbed dose calculations (weighted or unweighted) to be made. The majority of the calculated absorbed dose, for both marine and freshwater organisms, arises from internally incorporated alpha emitters, with 210Po and 226Ra being the major contributors. Calculated doses are somewhat higher for freshwater compared to marine organisms, and the range of doses is also much greater. This reflects both the much greater variability of radionuclide concentrations in freshwater as compared to seawater, and also variability or uncertainty in concentration factor values. This work has revealed a number of substantial gaps in published empirical data especially for European aquatic environments.
We analyse transaction logs for a large full-text document collection for Computer Science researchers. We report insights gained from this analysis and identify resulting search interface design issues.
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