Uncertainties and Limitations

There are a number of considerations that limit the accuracy and reliability of the approach outlined in this chapter. To begin with, as indicated above in the 

This may not be a good assumption with respect to conformational heat capacity because there is evidence suggesting that polymers adapt different conformations (which may have different energies) in the vicinity of nanotubes. Our calculations do account for this possibihty because the structure of the polymer was optimized independently with and without the nanotube, but we did not investigate whether these conformations change as a function of temperature. Thus, we are assuming that the structures obtained from our molecular mechanics calculations are representative of the structures that are populated at the processing temperature of the mixture. The reasonableness of this assumption was discussed in Section 4.2. 

Although the value for is in excellent agreement with the results obtained with 12 chains (Table 4.2), which implies that the limit of infinite dilution has been reached, fhe value for A pp is much differenl (fhe absolute value is almosf 60% too large) than the values we obtained from the smaller models (Table 4.2). The source of the disparity is that in these calculations (i.e., on the models with 

FIGURE 4.7 Nanotube (3.6 nm long) surrounded by 18 PS chains, each consisting of 17 monomers. 

Unfortunately, the calculation of the diameter for which the energy of mixing nanotubes in PS becomes exothermic is extremely sensitive to the errors in the component terms discussed above. Thus, when we attempted to correct for the errors in the forcefield by scaling the calculated values for and AE p by 0.7 

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Information or sources of uncertainties and limitations are available in tlie literature. Some of this material is provided below. 

A relatively large number of indoor radon measurements have already been made, particularly during the last 6-12 months. However, most of these measurements are not useful in defining the national frequency distribution of indoor radon levels because there are a number of uncertainties and limitations associated with these measurements. These uncertainties include the different purposes for the measurements, the widely varying sample designs, and the many different sample collection and measurement procedures used. 

In the final phase of risk analysis—risk characterization—one integrates outputs of effects and exposure assessments. Risk is expressed in qualitative or quantitative estimates by comparison with reference values (e.g., hazard quotient). The severity of potential or actual damage should be characterized with the degree of uncertainty of risk estimates. Assumptions, data uncertainties and limitations of analyses are to be described clearly and reflected in the conclusions. The final product is a report that communicates to the affected and interested parties the analysis findings (Byrd and Cothern, 2000). 

Other uncertainties and limitations Use professional judgment to determine a factor which is >0 and <10. The size of this factor depends on uncertainties in the study and database not explicitly treated in uncertainty factor. Default value is one. 

Despite the successes of the above mentioned techniques for structure and bond energy analyses, inherent uncertainties and limitations in each method make it important to develop new and independent tests for comparison. In this regard our laboratory has initiated an intensive effort to study the gas-phase photodlssocia-tion of transition metal containing ions [13-16]. 

Accuracy of information is a result of scientific expertise, the delivery of adequate, complete and unbiased information about results and residual uncertainties. The speed of release is influenced by the organizational culture, to what extent the process to find answers and to acknowledge uncertainties is developed. Empathy is related to the willingness to recognize the situation (the scenario) in which the persons/clients are found. The degree of openness corresponds to the information given about uncertainties and limitations in the exposure assessment, the restrictions with respect to selected scenarios, the model assumptions and... 

Recommendations Plant measurements should be adjusted to close the constraints of the process. This adjustment should be done on a component or subcomponent (e.g., atomic) basis. The adjustments should be done recognizing (at a minimum) the uncertainty in the measurements. While sophisticated routines have been developed for reconciliation, the vagaries of plant measurements may make them unsuitable in most applications. The routines are no substitute for accurate, precise measurements. They cannot compensate for the uncertainties and limited information typically found in plant data. 

With the above-mentioned uncertainties and limitations concerning the accuracy of commercial simulators, results obtained with Aspen, which can be used to at least directionally, illustrate the merits of using methane/diesel blends for a particular set of conditions with a diesel fuel at room temperature (70°F), are illustrated in Table 6. 

Given the differences, uncertainties, and limitations of the exposure-assessment approaehes used in the Seychelles, Faroe Islands, and New aland studies, none of the approaches can be identified as better or more relevant. It is clear, however, that each of the approaches supplied different, and not necessarily comparable pietures of exposure and dose. Grandjean et al. (1999) noted in the Faroe Islands study that cord-blood MeHg appeared to better predict deficits in cognitive functions (language, attention, and memory), and maternal-hair MeHg appeared to... 

The uncertainties and limitations in exposure assessment in these studies can result in exposure misclassification, which will lessen the ability to detect significant dose-response associations and might result in inaccuracies in the derivation of dose-response relationships. 

The application of uncertainty factors in the revision of the RfD should be based on a thorough quantitative and qualitative evaluation of the full range of uncertainties and limitations of the critical smdies. Uncertainty factors apphed in the development of a revised RfD should include data-base insufficiency and interindividual toxicokinetic variabihty in dose reconstmction. As a starting point, an uncertainty factor of 2-3 should be applied to a central tendency estimate of dose derived from maternal hair, or a factor of about 2 should be applied to a central tendency estimate of dose derived from cord blood to account for inlerindividual pharmacokinetic variabihty in dose reconstractioa The choice of an uncertainty factor for data-base insufficiency is, in part, a policy decision however, given the data indicating possible low-dose sequelae and latent effects and immunotoxicity and cardiovascular effects, the... 

There are only a few general criteria of validation that can be given for an analytical method. Each method has some specific criteria for validation. The general criteria must be related to both method and sample (especially concerning the type of matrix and provenance). The criteria referring to the method are related to selectivity, sensitivity, uncertainty, and limit of detection. 

Phenols are convenient substrates for the synthesis of coumarins, but the uncertainty and limited appHcability of the Pechmann reaction is a disadvantage. The effect of substituents on the mode of cyclization has been investigated and n.m.r. spectroscopy has been used to correlate orf/io-proximity effects with those in benzenoid compounds. Some phenols do not react under Pechmann conditions but do so when the keto-ester is replaced by tri-ethyl orthoacrylate (190). ... 

The uncertainties and limitations of these models should be identified and eliminated by comparison with monitoring data. 

Mann, M., Bradley, R., and Hughes, M. (1999). Northern hemisphere temperatures during the post millenium inferences, uncertainties, and limitations. Geophys. Res. Lett. 26,759-762. 

Mann, M.E., R.S. Bradley, and M.K. Hughes. (1999). Northern Hemisphere Temperatures During the Last Millennium Inferences, Uncertainties and Limitations. Geophysical Research Letters, 26, 759-762. 

The International Organization for Standardization (ISO) Guide 34 2000(E) [7] details that reference materials should also be characterized to the level of accuracy required for its intended purpose (i.e., appropriate measurement uncertainty) and limitations due to the material matrix should be known. 

Simiu, E. Probabilistic models of extreme wind speeds uncertainties and limitation. Proceedings of the 4th International Conference on Wind Effect on Structures and Buildings, London, England, September 1975, pp. 53-62. 

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