Population homogeneity

Estimates of responses at low doses derived from data on laboratory animals and extrapolated to humans are complicated by a variety of factors that differ among species and potentially affect the response to hazardous chemicals. These factors include differences between humans and experimental test animals with respect to life span, body size, genetic variability, population homogeneity, existence of concurrent disease, such pharmacokinetic effects as metabolism and excretion patterns, and the dosing regimen. These factors are discussed further in Section 3.2.1.5. 

In all treatment sets, and for each of the triplicate replicates, 3 times 50 mL of algal suspension (total volume 4 L) were taken at the end of the dark period and the assayed parameters subjected to Tukey s multiple range test (Tukey 1949) and Cochran s population homogeneity test (Cochran et al. 1941). 

If the relevant physical lesions are formed in direct proportion to dose, and if no dose-dependent processes are involved in the conversion of initial lesions to biological hits, then the hit function will be linear. More generally, H x) could be some more complex function that, nonetheless, can always be represented by an infinite power series in x with no constant term (since there can be no induced hits for zero dose). We denote the expected number of lethal and mutational hits at dose x by Hk x) and Hm(x), respectively. Thus, on the basis of single-event Poisson statistics, population homogeneity, and stochastic independence of mutation and killing, we can write... 

In practice, the kinetic pattern can sometimes be guessed from the shapes of S x) and M x) for the dose range studied. Thus, the experimentally determined value of x can be substituted directly into the inferred form of K(a ) to see whether the observed and predicted yield maxima agree. Any marked disagreement between the observed and predicted yield maxima would mean either that the kinetic pattern was not inferred correctly because the dose range studied was too limited or that the assumptions of population homogeneity and/or stochastic independence of mutation and killing do not hold for the system under consideration. 

The binomial distribution describes a population whose members have only certain, discrete values. A good example of a population obeying the binomial distribution is the sampling of homogeneous materials. As shown in Example 4.10, the binomial distribution can be used to calculate the probability of finding a particular isotope in a molecule. 

Few populations, however, meet the conditions for a true binomial distribution. Real populations normally contain more than two types of particles, with the analyte present at several levels of concentration. Nevertheless, many well-mixed populations, in which the population s composition is homogeneous on the scale at which we sample, approximate binomial sampling statistics. Under these conditions the following relationship between the mass of a randomly collected grab sample, m, and the percent relative standard deviation for sampling, R, is often valid. ... 

The initial sample is called the primary, or gross sample and may be a single increment drawn from the target population, or a composite of several increments. In many cases the gross sample cannot be analyzed without further treatment. Processing the gross sample may be used to reduce the sample s particle size, to transfer the sample into a more readily analyzable form, or to improve its homogeneity. 

Homogeneous exposure group A population or group of workers with similar exposure. 

The coalescence-redispersion (CRD) model was originally proposed by Curl (1963). It is based on imagining a chemical reactor as a number population of droplets that behave as individual batch reactors. These droplets coalesce (mix) in pairs at random, homogenize their concentration and redisperse. The mixing parameter in this model is the average number of collisions that a droplet undergoes. 

The population balance in equation 2.86 employs the local instantaneous values of the velocity and concentration. In turbulent flow, there are fluctuations of the particle velocity as well as fluctuations of species and concentrations (Pope, 1979, 1985, 2000). Baldyga and Orciuch (1997, 2001) provide the appropriate generalization of the moment transformation equation 2.93 for the case of homogeneous and non-homogeneous turbulent particle flow by Reynolds averaging... 

Using dose-response information from homogeneous animal populations or healthy human populations to predict the effects likely to be observed in the general population consisting of individuals with a wide range of sensitivities... 

Non-homogeneous CA. These are CA in which the state-transition rules are allowed to vary from cell to cell. The simplest such example is one where there are only two different rules randomly distributed throughout the lattice. Kauffman [kauff84] has studied the otlier extreme in whidi tlie lattice is randomly populated with all possible Boolean functions of k inputs. 

Non-Homogeneous CA a characteristic feature of all CA rules defined so far has been that of homogeneity - each cell of the system evolves according to the same rule 0. Hartman and Vichniac [hartSfi] were the first to systematically study a class of inhomogeneous CA (INCA), in which the state-transition rules are allowed to vary from cell to cell. The simplest such example is one where there are only two different 0 s, which are randomly distributed throughout the lattice. Kauffman has studied the other extreme in which the lattice is randomly populated with all 2 possible boolean functions of k inputs. The results of such studies, as well as the relationship with the dynamics of random, mappings, are covered in detail in chapter 8.3. 

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. 

Formation from Template Surfaces Recently, a new method for the preparation of LUV was reported by Lasic et al. (1988). The method is based on a simple procedure which leads to the formation of homogeneous populations of LUV with a diameter of around L vim. Upon addition of solvent to a dry phospholipid film deposited on a template surface, vesicles are formed instantly without any chemical or physical treatment. The formation of multilamellar structures is prevented by inducing a surface charge on the bilayers. The size of the vesicles is controlled by the topography of the template surface on which the phospholipid film was deposited (Lasic, 1988). 

Homogeneous molecular population for a wide range of macromolecules presenting variety of molecular weights and shapes ... 

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