Rare homogenity

A difficulty that should not be overlooked is that airborne particulates are rarely homogeneous. They vary greatly in size and shape, and their chemical composition is determined by factors specific to the source and location of the emissions. The combined effects and interactions of various substances mixed with particulates have not yet been established (except for sulfur dioxide), but they are believed to be significant, especially where long-term exposure occurs. Measurement techniques and their reliability may vary across regions and countries, and so may other factors, such as diet, lifestyle, and physical fitness, that influence the human health effects of exposure to particulates. 

A key element in planning and conducting clinical trials is to ensure that they have scientific validity and objectivity. This is particularly relevant with respect to Phase II and III studies, where it is desired to demonstrate a positive benefit to risk outcome. Responses to a drug among a patient population are rarely homogeneous and clear-cut. Thus, sound statistical principles must be applied in order to be able to distinguish significant effects from random events. 

A ceramic is rarely homogeneous, a common inhomogeneity being porosity. There is strong evidence to show that breakdown can be initiated at pores and that the occurrence of gas discharges within pores is an important factor. There... 

Uniform attack on a metal results in uniform corrosion. This is exploited in the processing and finishing of metals (Chapter 15). However, metallic structures are rarely homogeneous and surfaces are rough corrosion occurs, preferentially within fissures in the surface (crevice corrosion), making corrosion faster. 

When needle-shaped phases, such as the W 03 2 CS structures are considered, the fault planes run parallel to the needle axis. In addition, the needles are rarely homogeneous, and often consist of domains in which the planar defects are relatively well ordered, but oriented differently from domain to domain. It is this feature which has made these crystals so difficult to study by A -ray diffraction. No mechanism has been proposed in the literature to account for the formation of such materials, and so they will not be considered further here. 

The timing of the stressor s interaction with the biological system is another important consideration. If the stressor is episodic in nature, different species and life stages may be affected. In addition, the ultimate distribution of a stressor is rarely homogeneous it is important to quantify such heterogeneity whenever possible. 

Chemical interactions at the solid phase may comprise (i) formation or rupture of a bond between sorbate and surface (ii) further reaction between adsorbed species and, (iii) rearrangements of the solid structure and formation and disappearance of solid species. It is often incorrect to apply simple kinetic models such as first- or second-order rate equations to such interactions because reacting solid surfaces are rarely homogeneous and because effects of transport phenomena and chemical reactions are often experimentally inseparable (Sparks, 1989). 

A further problem is that a material is rarely homogeneous. The choice of the source material for the specimen will largely depend upon the aims of the experimental work. When studying corrosion behaviour for practical applications, samples taken from components or material with a similar production history are preferable. On the other hand, if material is simply taken from the next available source of material of nominally the same specification, it may not be truly representative of the material as used in practical applications. 

To obtain an unambiguous characterization of a particular material, it is often essential to fractionate a material (1-3). Synthetic polymers are rarely homogeneous chemical species, but have multivariate distributions in molecular weight, chemical composition, chain architecture, and functionality (4). For a precise characterization of a synthetic polymer, all the distributions need to be determined, which is a difficult, if not virtually impossible, task. Traditionally, fractionation has allowed separation of pol5miers on the basis of molecular mass or chemical composition (2). With proper techniques it is often possible to separate and characterize complex homo- and copolymer species on the basis of chemical heterogeneity and molar mass. 

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