Representativeness of samples

The critical weight of samples can be derived from the general condition of representativeness of sampling expressed by the null hypothesis H0 °totai = °anai which is tested by means of Fisher s F-test... 

The test set method of validation is rather straightforward, but requires some caution. First, the test set samples must be sufficiently representative of samples that the model will be applied to in the future. Otherwise, external validation can provide misleading results - in either an optimistic or pessimistic manner. Under-representing the anticipated sample states could lead to optimistic results, while the use of sample states not represented in the calibration data could lead to pessimistic results. A practical disadvantage of the test set method is that it requires that the reference analytical method be performed on an additional set of samples beyond the original calibration samples, which can be an issue if the cost of each reference analysis is high. 

The representativeness of samples is a very important premise for true assessment of the state of air pollution. The following example demonstrates the temporal frequency necessary for objective assessment of the impact of emissions of total suspended dust at one sampling location [GEISS et al., 1991] ... 

Results in Table III suggest that the dynamics of the Pu species in solution is very complex. In general, there appeared to be an increase in the concentration of Pu(VI) with time. For example, for sample 2 which is representative of samples being studied over an extended time (months), the Pu(VI) concentration increased from 0.38 x 10 10 M at 190 hours of contact to 1.2 x 10 10 M after 672 hours of contact. The quantity of Pu(IV) was... 

To retain valid representativeness after collection, samples must be handled and preserved using methods adequate for preventing changes in the concentration of materials to be analyzed, by loss or by introduction of outside contamination. Failure to take account of each of the factors which can potentially reduce the representativeness of samples for the target environment is likely to result in analyzing samples which are not truly representative. 

Representativeness of samples must be assured (Quevauviller 1995), and suitable sample containers must be used and carefully pre-deaned. The volume/surface ratio should be high in order to reduce adsorption or other effects of the container walls. If species oxidation is expected to occur, sampling should be performed in an inert gas atmosphere. Steel needles are not suitable for blood sampling (especially those with narrow ID) due to contaminating breakage of erythrocytes. Urine samples should be taken from a mid-stream urine flow. 

Sample injection is an important issue in forensic analysis as there should be a semi-quantitative relationship between peak height and sample concentration. This relationship helps the analyst assess the quality of the sample and its preparation. It also helps define the relative level of different contributors to a mixture. The removal of salts through dialysis or the unintentional addition of higher quantities of salt can produce peak intensities that are less representative of sample quantity and affect the aforementioned relationship. 

In addition to evaluation of new methods (how well do the analyses agree with each other as well as with older methods), or new people s ability to follow established methods (how well do student data agree with it), questions about the validity (i.e., representativeness) of samples must be addressed not only by analytical chemists, but all experimental scientists and engineers, by epidemiologists, marketers, journalists, politicians, and others interested in opinion surveys. 

A rather crucial factor for the credibility of the results is the representativeness of sampling and the mass of test portion. The mass of test portion required depends on the maximum size of the soil particles. For example, the minimum mass of test portion normally used for a maximum soil particle size of 9.5 mm is 0.5 kg and that for a maximum soil particle size of 75 mm is 5 kg. 

The representativeness of samples is directly related to the quantity taken to conduct the particular test. The sampling location does not affect the sample representativeness, since all sampling locations are theoretically equivalent. 

Monitoring activities should be such as to provide the necessary data for the analysis and evaluation of environmental contamination. Monitoring, especially in emergencies, is an important information source for decision making and for the justification of countermeasures. However, as with any measurements, monitoring data have associated uncertainties that arise from technical uncertainties, the non-representativeness of samples and/or measurements, and human errors. 

The variability of environmental factors and the representativeness of sampling and measurements ... 

Principles and Characteristics In general terms, quantitative polymer/additive analysis is a multi-analyte problem, which is even more complicated by the heterogeneity of some formulations, the representativity of sampling of the various analytical techniques, the presence of technical ingredients, the occurrence of degradation and... 

Concentrated acids that are representative of samples being run in the semiconductor industry... 

Carry out the method exactly as described in Section F.1-6, omitting the sea water and replacing it by about 5 ml of de-ionized water, the ammonia content of which is assumed to be negligible. The extinction should not exceed 0.3-0.35 on a 5-cm cell For this blank to be representative of samples of all salinities the assumption must be made that the sodium hydroxide solution introduces a negligible amount of ammonia. This appears to be justified with good quality alkali but the solution can always be boiled to remove ammonia if its presence is suspected. 

The electron microprobe results for samples reacted in dry and wet CO2 are shown in Figs. 9 and 10, respectively. The samples were comprised of 57-// quartz particles in a matrix of GE I CaCOa (2-3// vaterite) and are representative of samples shown in Fig. 8(//) and (6). The silicon and calcium traces shown in these figures represent concentration profiles for the respective elements for a beam traverse across the quartz particle, the product layer, and into the calcium carbonate bulk material. 

A suitable planning of sampling is the first and always crucial issue in scientific investigations. In particular, when dealing with development of geographical traceability models for PDO food, representativeness of sampling is certainly an imperative in order to obtain robust models and establish a food-territory link. 

Figure 3.3. Fluorescence fingerprint representative of samples collected in the (Arabian Sea) upwell-ing. Water Mass Type 4. Although typical of most samples collected in the cool upweUing waters, not aU had the chlorophyll-hke peak at ex/em = 400/685. (Reprinted from Coble et al., 1998, with permission from Elsevier.) Note that the shortest excitation wavelength was 260 nm therefore some of the peaks noted in Table 3.1 were not observed. The peak labeled A in the original figure is equivalent to peak A(, according to the revised nomenclature proposed in this work.

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