Minimising Uncertainties

Extra attention must be paid to minimising uncertainties but, even with all precautions, uncertainty of results is likely to be high. 

Kulik, D. A. 2002. Minimising uncertainty induced by temperature extrapolations of thermodynamic data A pragmatic view on the integration of thermodynamic databases into geochemical computer codes. Proceedings of the Workshop on The Use of Thermodynamic Databases in Performance Assessment , 29-30 May 2001, Barcelona, Spain. Organisation for Economic Cooperation and Development OECD, Paris, France, 125-137. 

Accurate solubility measurements are deceptively simple. The problem is essentially knowing whether equilibrium has been established. To minimise uncertainties it would appear that whenever possible, solubilities should be obtained by approaching the equilibrium state from both under- and super-saturated solutions. For the data to be thermodynamically useful, it is essential not only to obtain accurate solubilities, but also to determine the nature of the solid phase present. 

The determination of an acceptable dose for humans involves the application of uncertainty factors to reflect the fact that, unlike the experimental animal, there is wide variability and susceptibility of response in the genetically diverse human population. Variations in gender, age, hormonal and disease status can affect the response to a chemical. In order to minimise any potential risks, uncertainty factors are applied to the NOAEL to arrive at a reduced exposure that is considered tolerable - namely the acceptable daily intake or ADI. These are usually tenfold for variations in susceptibility amongst the human population (the intra-species factor) and tenfold for the potential... 

Chromatographic procedures applied to the identification of proteinaceous paint binders tend to be rather detailed consisting of multiple analytical steps ranging from solvent extractions, chromatography clean up, hydrolysis, derivatisation reactions, and measurement to data analysis. Knowledge of the error introduced at each step is necessary to minimise cumulative uncertainty. Reliable results are consequently obtained when laboratory and field blanks are carefully characterised. Additionally, due to the small amounts of analyte and the high sensitivity of the analysis, the instrument itself must be routinely calibrated with amino acid standards along with measurements of certified reference proteins. All of these factors must be taken into account because many times there is only one chance to take the measurement. 

A logical approach which serves to minimise such uncertainties is the use of a number of distinctly different analytical methods for the determination of each analyte wherein none of the methods would be expected to suffer identical interferences. In this manner, any correspondence observed between the results of different methods implies that a reliable estimate of the true value for the analyte concentration in the sample has been obtained. To this end Sturgeon et al. [21] carried out the analysis of coastal seawater for the above elements using isotope dilution spark source mass spectrometry. GFA-AS, and ICP-ES following trace metal separation-preconcentration (using ion exchange and chelation-solvent extraction), and direct analysis by GFA-AS. These workers discuss analytical advantages inherent in such an approach. 

The first is a matter of minimising variability, maximising the quantity of data and minimising the degree of extrapolation needed. For all but very modest extrapolations the uncertainty will inevitably be significant. 

Further uncertainty arises from the environmental exposure. For accelerated tests, repeatability can be estimated from exposure of replicate test pieces and minimised by control of the exposure conditions. Particular points to consider are spatial variation in temperature as well as mean temperature and air flow in ovens. In accelerated weathering apparatus, spatial variation and variation with time of light sources can be very significant. 

To summarise, while there is always considerable uncertainty in ED and no decision wiU be infallible, the risk of selecting the wrong compoimds for development can be minimised by identifying critical success factors and an MAP that will provide the basis for go/no-go decisions. 

From Table 3 the outermost electron-cloud radii of Cl and K+ from crystal data should be in the ratio 23.0/13.7 = 1.30. KCl is chosen for standard substance to minimise the two opposing sources of error uncertainty in the standard value for Li+ and uncertainty in the inner-shell correction. In KCl the structural nearest-neighbour distance is 3.147 A, hence Wasastjerna s Criterion so applied leads to the structural radii... 

In order to make the best possible use of valuable financial and other resources, and to minimise the uncertainty involved in making irreversible decisions, appraisal techniques are vitally important. These can be technical, legal, regulatory, commercial or financial in nature, for instance market forecasting and patent protection. Critical path analysis is... 

Absorbance measurements for solutions are carried out in cells with a small optical path d to minimise absorption of the solvents, none of which are transparent in this spectral domain. Uncertainty in the value of d is related to the fragility of the material used for the cell windows and their construction. Therefore, it is necessary to periodically calibrate the optical path. 

To eliminate these uncertainties about surface chemistry, further exeriments were performed with TiN in a N2 + H2 atmosphere. The role of H2 was to suppress or minimise oxidation caused by the reaction 2TiN + (3/2)02 o Ti2C>3 + N2 by decreasing P02. The role of N2 was mainly to ensure a stoichiometric TiN surface. In these conditions, the contact angle of Cu on TiN was found to be 96 2° (van Deelen et al. 1998). This value corresponds to a Wa/(2erLV) ratio of 0.45 higher than for pure Cu on ionocovalent oxides (0.20) but lower than for Cu on WC or Mo2C( 0.98, Table 7.10). 

We have developed a protocol which describes how data generated from experimental studies commonly undertaken for method validation purposes can be used in measurement uncertainty evaluation. This paper has illustrated the application of the protocol. In the example described, the uncertainty estimate for three analytes in different oil matrices was evaluated from three experimental studies, namely precision, recovery and ruggedness. These studies were required as part of the method validation, but planning the studies with uncertainty evaluation in mind allowed an uncertainty estimate to be calculated with little extra effort. A number of areas were identified where additional experimental work may be required to refine the estimates. However the necessary data could be generated by carrying out additional analyses alongside routine test samples. Again this would minimise the amount of laboratory effort required. 

Four field techniques have been used to estimate the import/export of nutrients between saltmarshes and the adjacent coastal water community budgeting, direct creek studies, flume studies and diffusion chamber studies. The direct tidal creek method is the only one which takes account of the processes in all of the saltmarsh subsystems and hence appears to be the best technique, provided the uncertainties in water discharge are minimised. 

Long-term reproducibility is the prime quality of an analytical method used for the study of stability. Stability must be performed on the element or substance to be certified. Extrapolations on the stability of tracers are of little interest. If the uncertainty of the method for an analyte is poor, even for large sample intakes, the analyst must refer to the experience on similar materials with higher contents or to the general chemical properties of the substance or element. In any other situations certification may be impossible. Such difficulties are mainly encountered in organic or organo-metallic analysis. Experience has shown that some substances may be stable in a matrix but unstable in another even similar one [46-47]. All analytical methods suffer from long term reproducibility. In some cases analysts have developed tools and tricks to minimise this effect. 

Ingamell s equations show how the size of the sample needed to minimise sampling error is dependent on the maximum particle size of the sample, using a sampling constant Ks, the amount of sample required to ensure that the sampling uncertainty will not exceed a desired level assuming a precise analytical method (Vander Voet Riddle 1993). 

Sensitivity of biotests in terms of uncertainty considerations refers to the potential of a test system to indicate or to predict correctly the effects of contaminants in the sediment, thereby minimising false positive (nontoxic sample inconectly classified as toxic) and false negative (toxic sample incorrectly classified as nontoxic) results. Ingersoll et al. (1997) pointed out that whole-sediment tests with benthic organisms and acute measurement endpoints showed a high degree of certainty in this respect. This has also been shown by Ronnpagel et al. (1998) for a bacteria contact test which correlated well with the autochthonous microbial activity in experiments with spiked sediments. 

It is possible to use solid catalysts in particulate forms in tubular reactors through the use of fluidised or fluid bed reactors, where the upward flow of the feed is sufficient to suspend the particulate catalyst in such a way that it seems to behave like a liquid (Figure 1.3). It is however preferable to use more structured catalysts, since better flow characteristics can be achieved, thus minimising hydrodynamic uncertainties and maximising volumetric reaction rates. 

The object function is minimised with respect to p using a generalised gradient algorithm, yielding an optimised vector of parameters, p° The uncertainty in the values of the optimal parameters has been determined from ... 

---