Precision of sampling

Concentration assays are often the least demanding, since usually the component to be measured is abundant and minor components scarce. Even if resolution is poor or there is detector noise, accurate measurements of concentration can still be obtained. In concentration assays, the principal requirements are stringency in the precision of sample dilution and measurement of column losses of the major component. Detector calibration, another important issue in concentration assays, has been discussed above. 

Sample analysis data quality. Precision of sample analysis is almost always measured by determining the RSD at two or more concentrations without using a calibration curve. Such data do not include the effects of the calibration process on precision. Bluch better information is given by the relative confidence bandwidth (RGB) defined as ... 

Analysis of the collected fumes and determination of the overall accuracy and precision of sampling and analysis for each fume. 

In summary, the precision of sampling improves with the size of each of the increments collected and with the number of increments included in a gross sample and manual sampling involves the principle of ideal sampling insofar as every particle in the entire mass to be sampled has an equal opportunity to be included in the sample. 

Commonly the compromising conditions of routine environmental monitoring lead to restrictions on the accuracy and the precision of sampling and analysis. The purpose of this section is to show that under these conditions multivariate statistical methods are a useful tool for qualitative extraction of new information about the degree of stress of the investigated areas, and for identification of emission sources and their seasonal variations. The results represented from investigation of the impact of particulate emissions can, in principle, be transferred to other environmental analytical problems, as described in the following case studies. 

The RPD calculated for pairs of identical environmental samples (field duplicates) is the measure of total sampling and analysis precision, which combines the precision of sampling, sample handling, and the precision of sample preparation and analysis. Precision of field duplicates may be significantly affected by matrix interferences and by inherent sample variability. That is why the SAP should make a distinction between analytical precision determined from LCS/LCSD pairs and total sampling and analysis precision determined from field duplicate pairs and adopt separate acceptance criteria for each. 

The laboratory detects measurement errors by comparing the accuracy and precision of sample and laboratory QC check sample analysis to acceptance criteria. If results do not meet the criteria, the laboratory identifies the reason and takes corrective action, such as the following ... 

What determines the precision of sample estimates of a proportion ... 

The simple formula above is adequate for giving a basic impression of the calculations required to establish a sample size. In practice there are many complicating factors which have to be considered before such a formula can be used. Some of them present severe practical difficulties. Thus a cynic might say that there is a considerable disparity between the apparent precision of sample size formulae and our ability to apply them. 

The need for accuracy in a workplace monitoring method is vital, since the measurements need to reflect the actual conditions in the workplace. On the other hand, given the temporal and spatial variability in most airborne contaminant concentrations in the workplace, it is generally not required that the monitoring method be highly accurate, i.e. within a few percent of the true value . The NIOSH (USA) recommends that (1) the overall bias of a monitoring method be < 10% of the values determined by a well-characterized independent method, and (2) that the overall precision of sampling and analysis should be such that the total error is <25% in at least 95% of the samples analyzed, based on the analysis of 6-10 samples at each of three or four concentration levels. 

Now becanse C, the total count, can never be less than B, then Atg, the time devoted to background counting, should never be greater than Atg, or otherwise the precision of sample measurement will suffer. For a sample of four times background, we would achieve the best precision if we counted the sample for two thirds of the available time and the background for one third. 

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