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This paper evaluates the reliability of variants of five urban shopping models : Central Place theory, the Intervening Opportunities model, Gravity model, the Entropy model, and the Building Research Establishment (BRE) model . For each model variant parameters are estimated against data for four towns in the United Kingdom : Coventry, Derby, Watford and Telford, using alternative procedures for testing significance of the calibration . The models are then used to forecast hypothetical changes in local shopping environments, including dynamic effects, such as the progressive decline of local centres . On the basis of the results, some general conclusions are drawn as to the relative performance of the models and the factors determining their reliability in different forecasting situations.Mathematical models are used extensively in urban and regional planning to estimate traffic flows, housing demand and the likely impact of new shopping amenities . There is still, however, considerable controversy surrounding the use of models and great suspicion about the reliability of their results . Very rarely, in fact, have the forecasts made using these models been checked . In this paper we take shopping models as a typical example of the problems and possibilities with regard to the use of models in general .Other urban models (such as the traffic forecasting methodology of the Transport and Road Research Laboratory) and their predictions have been more thoroughly criticised (Gershuny, (1978) ; Turner, (1979, forthcoming) and comparisons of existing spatial interaction models made by Openshaw (1976) and Hathaway (1975)) . Comparisons of urban shopping models (e .g. Hodges and Kennett, 1973 ; Pankhurst and Roe, 1978) are referred to later . Models often conceal, in their wealth of technicalities, decisions that could substantially affect the results of using them (see, for example, Massey and Cordey Hayes, 1971) . In particular, the application of a given shopping model may lead in particular instances to quite arbitrary results . This of course has significant implications for any conclusions based R .on these models . The aim of the paper is to obtain an understanding of how reliable such models and the procedures for using them may be and how models trade off the interests of different social groups, as affected by planning decisions . Following this, we make suggestions about the use of models in planning . Although the discussion is for foodstuffs shopping models, the general arguments apply to other kinds of urban and regional models . We do not intend to consider the technical problems of building a model in any detail here . Rather, in the various stages of the construction and application of the models we shall indicate where uncertainty may arise, or judgments are made about the content of the model . These occur especially in the collection of data and their measurement, the choice of model, the form of the relationship used in a model, the procedure used to fit the data to the model (i .e . calibration) and t...

API RP2A-LRFD[l] is soon to be adopted by API for the design of fried offshore steel platforms, and will be available for use worldwide. The paper describes the work under taken to adapt LRFD for use in the North Sea. Sets of partial safety factors have been calibrated to suit specific environmental conditions, and design and operating practices using a reliability-baaed optimisation procedure. INTRODUCTION API RP2A[2] (henceforth denoted RP2A-WSD for Working Stress Design) forms the basis of steel jacket design throughout the world. It is widely used and has proved a popular design standard since fiit issued in October 1969. API have continually revised and updated it as a matter ofpolicy to include both new information and research, and to reflect current trends. RP2A-WSD is now on its 19th Edition. Throughout most branches of structural engineering theinternational trend in design standards is to move from the working stress, or permissible stress, design format, typified by RP2A-WSD, towards a multiple factor format with partial safety factors applied to both loads or loading effects, and toresistances. By careful adjustment or calibration of the partial factors to reflect the uncertainty and variability in the different types of loads and resistances a much more consistent level ofcomponent reliability can be achieved. The partial factor format also offers more opportunity for influencing and improving design. In 1979 API set up the PRAC-22 Committee to develop a partial factor format for RP2A; their work is reported inreference [3]. Following considerable development a draft Recommended practice 2A-LRFD was finally published in December 1989 and has been available for comment. The two year review period has now ended and RP2A-LRFD is soon expected to be issued as an API Recommended Practice. In common with RP2A-WSD, LRFD has been written anddeveloped for use specifically in the Gulf of Mexico, the waters off Alaska, and the Pacit3cand Atlantic seaboards of the USA. When RP2A has been specified for design use in other areas of the world it has been subject to regional modifications and supplemented by additional guidance. When LRFD is internationalised it is hoped and intended that regional modification will be incorporated into Annexes for specific areas. In addition to area specific guidance, modified partial factors may be included which account for regional differencein environmental conditions, and in design and operating practices. This ability to tailor the safety factors to suit specific conditions is a major advantage over the working stress approach and should have significant benefits for platform design and the internationalisation of RP2A-LRFD. The calibration of the partial factors is undertaken using a reliability-based procedure. The general principles of the code calibration procedure are widely accepted. Firstly a set of one or more target reliabilities are assessed from a range of representative components which have been designed to an swept.ed code (usually the previous WSD version).

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