Network practical issues

EURACHEM is a network of organizations in Europe, having the objective of establishing a system for the international traceability of chemical measurements and the promotion of good quality practices. It provides a forum for the discussion of common problems and for developing an informed and considered approach to both technical and policy issues. 

Independent studies (Cybenko, 1988 Homik et al., 1989) have proven that a three-layered back propagation network will exist that can implement any arbitrarily complex real-valued mapping. The issue is determining the number of nodes in the three-layer network to produce a mapping with a specified accuracy. In practice, the number of nodes in the hidden layer are determined empirically by cross-validation with testing data. 

Of course, there are limitations to this approach that must be kept in mind. First, because we use commercially available components for the data visualization, we are tied to the standard file formats. For the most part these formats are sufficient, but in certain cases they can be limiting. For example, there is no standard for handling multiple entries for a single data field. This becomes an issue when retrieving assay results, because in many cases more than one experiment has been run. Second, this system requires a robust and fast network since many file uploads and downloads to the server are involved. However, in practice we have found these issues to be manageable. 

In practice decision makers typically are risk averse and the expected value approach does not take into account the variability of the solutions obtained under the probability distributions or scenarios considered for the uncertain parameters. Rosenhead et al. (1972) introduced the aspect of robustness as a criterion for strategic planning to address this issue. Building on the notion of robustness, Mulvey et al. (1995) developed the concept of robust optimization distinguishing between two different types of robust models. A model is solution robust if the solution obtained remains close to optimality for any realization of the uncertain parameters. The model itself is robust if it remains (almost) feasible for any realization of the uncertain parameters (model robust).36 Here, only solution robustness is of interest as the most important elements of uncertainty in production network design, namely demand volumes, costs, prices and exchange rates, should not lead to infeasibility problems under different scenarios considered. 

Another issue is related to spreading viruses in the Internet. It is not entirely clear that in practice the spread of viruses is intensified by hubs. Viruses target individual computer systems or companies not hubs. Interesting network issues have been mentioned by Peter Denning [11]. He points out that network theories can help us to understand how innovations spread and how modem network-centric military and industrial organizations function. 

The User Interface. A wide variety of stand alone steady state simulation programs and flowsheet systems are available to the process engineer. These have been reported in a series of articles by Peterson, Chen and Evans in 1978 (j>) and by Chen and Evans in 1979 (6). Some practical advice on the use of the computer Tn design is reported by Weismantel (7 ). A course in the use of several commercially available systems is given in the AIChE Today Series ( ). A report on the use of networks to share chemical engineering programs among educators was recently issued (9 ). ... 

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