Model ambiguities

Given the elongated shape of the ICAM-1 D1D2 model, ambiguity exists in its correct orientation around the major D1D2 axis. That problem can be solved by comparing EM... 

However, using the test set mean can significantly underestimate the predictive ability of a model. Ambiguity occurs with the statistic when the test set data is not evenly distributed over the range of the training set. As the variance of the external test set approaches the RMSE of the fitted model, the measure would approach zero, even though it would appear that the predictions are in accordance with the model. Consonni defined a new statistic that expresses the mean predicted error sum of squared deviations between the observed and predicted values for the test set, over the mean training set sum of squared deviations from the mean value ... 

In an isolated two-spin system, the NOE (or, more accurately, the slope of its buildup) depends simply on where d is the distance between two protons. The difficulties in the interpretation of the NOE originate in deviations from this simple distance dependence of the NOE buildup (due to spin diffusion caused by other nearby protons, and internal dynamics) and from possible ambiguities in its assignment to a specific proton pair. Mofec-ufar modeling methods to deaf with these difficulties are discussed further below. 

The reason for that ambiguity is the fact that the l/V characteristics are Fitted with equations (see above) consisting of various fit parameters in a rather restricted voltage range, so that many different curves can model the l/V curves within a moderate variance range. 

The issue of stereochemistry, on the other hand, is more ambiguous. A priori, an aldol condensation between compounds 3 and 4 could proceed with little or no selectivity for a particular aldol dia-stereoisomer. For the desired C-7 epimer (compound 2) to be produced preferentially, the crucial aldol condensation between compounds 3 and 4 would have to exhibit Cram-Felkin-Anh selectivity22 23 (see 3 + 4 - 2, Scheme 9). In light of observations made during the course of Kishi s lasalocid A synthesis,12 there was good reason to believe that the preferred stereochemical course for the projected aldol reaction between intermediates 3 and 4 would be consistent with a Cram-Felkin-Anh model. Thus, on the basis of the lasalocid A precedent, it was anticipated that compound 2 would emerge as the major product from an aldol coupling of intermediates 3 and 4. 

Appreciable deviations from the assumed simple model with constant Ed and kd throughout a significant desorption range evidently impair the estimate of the desorption order and may make its value quite ambiguous. 

Triad information is more powerful, but typically is subject to more experimental error and signal assignments are often ambiguous (Section 7.3.3.12). Triad data for the MMA-S system are consistent with the terminal model and support the view that any penultimate unit effects on specificity are small.Mv lS... 

A sutmnaty of the above shows various terms used for eaeh type of representation first (maero level, maeroscopic level, macroscopic world), second (sub-micro level, microscopic level, submicro level, submicroscopic level, molecular world, atomic world), and third (symbolic level, sy mbolic world, representational chemistry, algebraic system). In onr view, the system of terminology shonld be both as brief as possible and avoid any possible ambiguities of meaning. Conseqnently, sub-micro and snb-microscopic fall foul of our first criterion for they perhaps imply that snch a level can be seen through an optical microscope. For those reasons, we have decided to nse macro, submicro, symbolic for the individual types and triplet relationship to cover all three. The triplet relationship is a key model for chemical edncation. However, the authors in this book have been fiee to decide for themselves which conventions to use. Nevertheless, it is our intention to promote the terms macro, submicro, symbolic in all subsequent work and to discuss the value of the triplet relationship in chemical education. 

The quality of front-end work in answering the questions above, when addressed by a broadly comprised project team, is directly related to project success. It is useful to keep in mind that the consequences of decisions (question 4) are often different for the firm, for departments, for research groups, and for individuals on the research team. Models can reduce ambiguity and arbitrary discretion. This is appreciated in successful projects. 

The picture of cement microstructure that now emerges is of particles of partially degraded glass embedded in a matrix of calcium and aluminium polyalkenoates and sheathed in a layer of siliceous gel probably formed just outside the particle boundary. This structure (shown in Figure 5.17) was first proposed by Wilson Prosser (1982, 1984) and has since been confirmed by recent electron microscopic studies by Swift Dogan (1990) and Hatton Brook (1992). The latter used transmission electron microscopy with high resolution to confirm this model without ambiguity. 

Two comments can be made on the second point. For a simple mathematical reason mistakes made with the LEL value are of little consequence to the calculated value of flashpoint cc . Indeed, this mistake is not that significant since there Is a logarithm involved. Secondly, in theory no mistake is made with the stoichiometric concentration (except for nitrogenous compounds where there is an ambiguous aspect with regard to the nitrogen reaction). This second approach (with Cg) can thus provide preliminary control of the model parameters (S or the group) and there... 

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