SELECTION OF REFERENCE INDIVIDUALS

A set of selection criteria determmes which individual should be included in the group of reference individuals. Such selection criteria include statements describing the source population, specifications of criteria for health, or the disease of interest. 

separate reference values for each sex, different age groups, and other criteria are necessary. Our group of reference individuals may therefore have to be divided into more homogeneous subgroups. For this purpose, we need to specify rules for the division, called stratification or partitioning criteria. 

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Several methods have been suggested for the selection of reference individuals. Table 16-1 shows three pairs of concepts that may be used to describe a sampling scheme. The concepts of each pair are mutually exclusive. For example, the sampling is either direct or indirect. One may, however, combine one concept from several pairs to obtain a more exact description. For example, the selection may be direct, a posteriori, and nonrandom. 

Direct selection of reference individuals (see Table 16-1) is the only method that agrees with the concept of reference... 

The first step in the process of producing reference values for a laboratory test should always be the collection of quantitative information about the sources of biological variation for the analyte studied. A search through relevant literature may give the required information (see Chapter 17). If the relevant information cannot be found in the fiterature, pilot studies may be necessary before the selection of reference individuals is planned in detail. 

It is important to distinguish between controllable and noncontrollable sources of biological variation. Some factors may be controlled by the standardization of the procedure for preparation of reference individuals and specimen collection (see a later section of this chapter). Other factors, such as age and gender, may be relevant partitioning criteria, The remaining sources of variation should be considered when defining the criteria for the selection of reference individuals. 

The selection of reference individuals consists essentially of applying defined criteria to a group of examined candidate persons,The required characteristics of the reference values determine which criteria should be used in the selection process. Box 16-1 lists some important criteria to consider when the production of health-associated reference values is the aim. 

If, for example, we measure the concentration of triglycerides in sera collected from a group of reference individuals selected for comparison according to a sufficiently exact set of criteria, the triglyceride results are our reference values. 

It is important to realize that we do not obtain a random sample, in the strict sense, if we start by selecting individuals randomly from the entire population and then apply inclusion criteria to sort out the subset of individuals fulfilling these criteria, even though this may be the best approximation we can hope to obtain. Usually the situation is less satisfactory. A sample of reference individuals obtained by selecting among blood donors, persons working in a factory,... 

The first step in the establishment of reference values is the selection of a group of reference individuals. It is usually not feasible to obtain observations on aU possible reference individuals of a certain category of the general population. We therefore hope that the smaller group examined, the subset (sometimes called the reference sample group), can give us the desired information about the characteristics of the complete set of individuals (the reference population). 

Berg B, Nilsson JE, Solberg HE, et al. Practical experience in the selection and preparation of reference individuals Empirical testing of the provisional Scandinavian recommendations. In Grasbeck R, Alstrom T, eds. Reference values in laboratory medicine. Chichester, England John Wiley, 1981 55-64. 

Hence there is a need to make SOCI more computationally efficient so that it can be used for larger chemical systems, and to develop related methods which scale better with the system size. Although the Cl space can be reduced by individual selection of references or A-electron functions, for the reasons stated above it is beneficial to select the Cl space in an a priori manner, once a minimal set of parameters, such as the active space, has been specified. For example, we have advocated a method we call CISD[TQ],16-18 which is a SOCI in which higher-than-quadruple substitutions have been excluded. For systems dominated by a single reference, CISD[TQ] performs nearly as well as SOCI.16,17... 

The DRIs consist of four components. Each type of reference value is calculated from daily intakes averaged over time (usually one or more weeks). The surveys include, but are not limited to, (l)random selection of healthy individuals and asking them to either report what they have eaten or to maintain food diaries, (2) monitoring overall food production and consumption, and (3) correlating a defined population s health status with the group s food intake. Sometimes the results from the surveys are correlated with the type of assays listed in Table 8.2. The four Dietary Reference Intakes are ... 

The variety of molecules used to prepare LB films is enonnous. and only a small selection of examples can be presented here. Liquid crystals and biomolecules such as phospholipids, for example, can also be used to prepare LB films. The reader is referred to tire literature for infonnation about individual species. 

Several hundred types of Hquid phases are commercially available. These have been used individually or in combination with other Hquid phases, inorganic salts, acids, or bases. The selection of stationary phases for a particular appHcation is beyond the scope of this article, however, it is one of the most important chromatographic tasks. Stationary phase selection is discussed at length in books, journal articles, and catalogs from vendors. See General References for examples. 

Selection of appropriate time intei vals for increment extractions relates to property variation (inhomogeneity) within material flow streams. Ten minute extraction intei vals are generally adequate to obtain suitably representative samples from material flows under practical circumstances. Precise determination of extraction intei vals consistent with individual apphcations can be calculatedthrough autocorrelation of historical sampling data, a statistical method described in references (Gy, Pitard). 

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