Realism

The accuracy of absolute risk results depends on (1) whether all the significant contributors to risk have been analyzed, (2) the realism of the mathematical models used to predict failure characteristics and accident phenomena, and (3) the statistical uncertainty associated with the various input data. The achievable accuracy of absolute risk results is very dependent on the type of hazard being analyzed. In studies where the dominant risk contributors can be calibrated with ample historical data (e.g., the risk of an engine failure causing an airplane crash), the uncertainty can be reduced to a few percent. However, many authors of published studies and other expert practitioners have recognized that uncertainties can be greater than 1 to 2 orders of magnitude in studies whose major contributors are rare, catastrophic events. 

In true simulations physical realism is the goal, and the question arises. What part of such realism is sacrificed with the elimination of unimportant degrees of freedom This issue appears to be rather complicated. It has been debated many times in the literature, but no consensus seems to have been reached [6,9-16]. Without going into details, I briefly summarize here the two opposite lines of argumentation, denoting them (A) and (B). 

Generally these codes/models are limited in ability to incorporate all of the aspects of fire while still maintaining a simple physical description of how enclosure fires develop. This requires balancing mathematical detail against physical realism. 

Representativeness can be examined from two aspects statistical and deterministic. Any statistical test of representativeness is lacking becau.se many histories are needed for statistical significance. In the absence of this, PSAs use statistical methods to synthesize data to represent the equipment, operation, and maintenance. How well this represents the plant being modeled is not known. Deterministic representativeness can be answered by full-scale tests on like equipment. Such is the responsibility of the NSSS vendor, but for economic reasons, recourse to simplillcd and scaled models is often necessary. System success criteria for a PSA may be taken from the FSAR which may have a conservative bias for licensing. Realism is more expensive than conservatism. 

The problems that have been experienced in the recirculating rig test are indicative of those often met in performance testing. Attempts to reproduce the service conditions in a laboratory test inevitably involve attempting to reproduce each of the controlling conditions that exist in the real situation. Variations, which may be relatively small, in these simulations can lead to significant differences in test results. There is therefore much to be said for keeping test conditions as simple as possible rather than attempting to reproduce accurately the conditions in practice. A balance between reproducibility and realism has to be struck. 

This basic combat logic may be enhanced by three additional functions (1) defense, which adds a notional ability to agents to be able to withstand a greater number of hits before having their state degraded, (2) reconstitution, which adds a provision for previously injured agents to be reconstituted to their alive state, and (3) fratricide ( or friendly fire), which adds an element of realism to ISAAC combat by making it possible to inadvertently hit friendly forces. 

This is actually the pessimistic meta-induction associated with Larry Laudan among other philosophers. L. Laudan, A Confutation of Convergent Realism, Philosophy of Science, 48 1-49, 1981. 

The last elements of realism we will add to the data is random error or noise. In actual data there is noise both in the measurement of the spectra, and in the determination of the concentrations. Accordingly, we will add random error to the data in the absorbance matrices and the concentration matrices. 

Obviously, construction of a mathematical model of this process, with our present limited knowledge about some of the critical details of the process, requires good insight and many qualitative judgments to pose a solvable mathematical problem with some claim to realism. For example what dictates the point of phase separation does equilibrium or rate of diffusion govern the monomer partitioning between phase if it is the former what are the partition coefficients for each monomer which polymeric species go to each phase and so on. 

MERL s interest in employing these tests has included (1) the quantifying of D to enable certain predictions to be made (see Section 23.4.6), (2) testing at high temperature and pressures to simulate service realism (see Section 23.5.1.3), and (3) to provide data for use in estimating permeation rates while not needing to conduct actual permeation tests (see Section 23.4.4.3). 

The degree of realism of these model structures can be assessed by comparison of computed properties with experimental ones. The cohesive energy is, by definition, the difference in energy per mole of substance between a parent chain in its bulk environment and the same parent chain in vacuo, i.e., when all intermolecular forces are eliminated. This difference is readily computed from the minimized... 

A step closer toward realism is taken by off-lattice models in which the backbone is specified in some detail, while side chains, if they are represented at all, are taken to be single, unified spheres [44-50]. One indication that this approach is too simplistic was given in [51], which proved that for a backbone representation in which only Ca carbons were modeled, no contact potential could stabilize the native conformation of a single protein against its nonnative ( decoy ) conformations. However, Irback and co-workers were able to fold real protein sequences, albeit short ones, using a detailed backbone representation, with coarse-grained side chains modeled as spheres [49, 52-54]. 

At the turn of the century, this situation partly changed. By economic selection, only those developments that were market-oriented survived. In addition, most commentators in the field such as the trade press realized that they had to change their attitude on reporting a proper outlook, i.e. what can be realistically expected in the coming years hence, euphoria changed to (sober) realism [242]. 

There is a deluge of papers, as well as of methods and of approaches, addressed to these problems. It is significant that in this blossoming of studies there is space for very simple models (as regards the material composition of the model) as well as for very complex models with a high degree of realism in the chemical composition. 

Of course, whilst the aim of the modelling exercise is always to obtain as realistic a description of the process phenomena as possible, additional realism often involves additional numerical complexity and will demand additional data, which may be difficult or impossible to obtain. A marginal additional degree of realism can thus become rapidly outweighed by the large amount of extra time and effort needed. 

T. Harry Williams. Romance and Realism in Southern Politics. Athens University of Georgia Press, 1961 17-43. Source for the unification movement. 

Although all these codes have been tested for theoretical integrity and realism to some extent, few appropriate data sets for model testing are available and standardized methods for collecting them are only now under development (56). At this time, a full evaluation of a chemical is perhaps best accomplished using several fate codes, with careful comparisons among the outputs of the codes. 

The Leggett Model simulates the age-dependence of lead kinetics on such factors as bone turnover rates, partitioning between soft tissues and excreta, removal half-times in liver, kidneys, and red blood cells, and the deposition fraction in brain. The model structure represents a compromise between biological realism and practical considerations regarding the quantity and quality of information available to determine parameter values (Leggett 1993). 

Once the determinants of PPII helix formation are known in more detail, it will become possible to apply them, along with the known determinants of the cy-helical conformation, to the understanding of protein unfolded states. If, as suggested at the beginning of this article, protein unfolded states are dominated by residues in the PPII and cy-conformal ions, these data will allow for modeling of the unfolded state ensembles of specific proteins with a level of realism that has not been previously anticipated. 

Macdonald, G. T. (1992), Reduction and evolutionary biology , in D. Charles and K. Lennon (Eds), Reduction, Explanation and Realism, Clarendon Press, Oxford, UK, pp. 69-96. 

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