Measurement, operational definitions

Practical activities should embody as best as possible the scientifie proeesses that have been preseribed by the American Association for the Advancement of Science observation, elassification, numerieal relations, measurements, time-spaee relations, eommunieation (oral, pictorial, written), deriving of conclusions, prediction ( what would happen if. .hypothesis making, production of operational definitions, identifieation and control of variables, experiment and explanation of experimental data. Different theoretical perspectives should be used with the aim to optimize the positive eognitive and affeetive outcomes. The use, sometimes together, sometimes separately, of different perspeetives can act complimentarily and can lead to positive results (Niaz, 1993 Tsaparhs, 1997). 

Thermodynamics describes the behaviour of systems in terms of quantities and functions of state, but cannot express these quantities in terms of model concepts and assumptions on the structure of the system, inter-molecular forces, etc. This is also true of the activity coefficients thermodynamics defines these quantities and gives their dependence on the temperature, pressure and composition, but cannot interpret them from the point of view of intermolecular interactions. Every theoretical expression of the activity coefficients as a function of the composition of the solution is necessarily based on extrathermodynamic, mainly statistical concepts. This approach makes it possible to elaborate quantitatively the theory of individual activity coefficients. Their values are of paramount importance, for example, for operational definition of the pH and its potentiometric determination (Section 3.3.2), for potentiometric measurement with ion-selective electrodes (Section 6.3), in general for all the systems where liquid junctions appear (Section 2.5.3), etc. 

Under these conditions, an operational definition must be used, i.e. a definition based on the method by which the quantity pH is measured. Obviously, the operational definition must be formulated as closely as possible to the absolute equation (1.4.39). In this way, a practical scale of pH is obtained that is very similar to the absolute scale but is not identical... 

In whole tissue or cell monolayer experiments, transcellular membrane resistance (Rm = Pm1) lumps mucosal to serosal compartment elements in series with aqueous resistance (R = P ). The operational definition of Lm depends on the experimental procedure for solute transport measurement (see Section VII), but its magnitude can be considered relatively constant within any given experimental system. Since the Kp range dwarfs the range of Dm, solute differences in partition coefficient dominate solute differences in transcellular membrane transport. The lumped precellular resistance and lumped membrane resistance add in series to define an effective resistance to solute transport ... 

Shuman and Michael [10] applied a rotating disk electrode to the measurement of copper complex dissociation rate constants in marine coastal waters. An operational definition for labile and non-labile metal complexes was established on kinetic criteria. Samples collected off the mid-Atlantic coast of USA showed varying degrees of copper chelation. It is suggested that the technique should be useful for metal toxicity studies because of its ability to measure both equilibrium concentrations and kinetic availability of soluble metal. 

The scientific method begins with observation. Observation often demands careful measurement. It also requires the establishment of an operational definition of measurements and other concepts before the experiment begins. 

The operational approach to the definition of fundamental concepts in science has been emphasized by Mach, Poincare, and Einstein and has been expressed in a very clear form by Bridgman [2]. (Operational definitions had been used implicitly much earlier than the twentieth century. Boyle, for example, defined a chemical element in terms of the experiments by which it might be recognized, in order to avoid the futile discussions of his predecessors, who identified elements with qualities or properties.) In this approach, a concept is defined in terms of a set of experimental or mental operations used to measure or to recognize the quantity The concept is synonymous with the corresponding set of operations (Bridgman). An operational definition frequently may fail to satisfy us that we know what the concept really is. The question of scientific reality has been explored by many scientists and philosophers and is one that every student should examine. However, in the operational approach, we are not concerned with whether our definition has told us what the concept really is what we need to know is how to measure it. The operational approach has been stated succinctly by Poincare in the course of a discussion of the concept of force ... 

Relativity theory, with its rigorous operational definitions of time and space, led to many unexpected results that are contrary to common experience. One result was that the measured length of a body depends on the speed with which the body moves with respect to the observer. These new theorems from relativity theory removed apparent contradictions that had perplexed physicists in their measurements of the speed of light, and they also allowed prediction of a variety of new phenomena that since have been verified abundantly. 

Operational definition A property defined by the manner in which it is measured. Opposite of a theoretical or conceptual definition. 

The first satisfactory definition of entropy, which is quite recent, is that of Kittel (1989) entropy is the natural logarithm of the quantum states accessible to a system. As we will see, this definition is easily understood in light of Boltzmann s relation between configurational entropy and permutability. The definition is clearly nonoperative (because the number of quantum states accessible to a system cannot be calculated). Nevertheless, the entropy of a phase may be experimentally measured with good precision (with a calorimeter, for instance), and we do not need any operative definition. Kittel s definition has the merit to having put an end to all sorts of nebulous definitions that confused causes with effects. The fundamental P-V-T relation between state functions in a closed system is represented by the exact differential (cf appendix 2)... 

From the standpoint of the operational definition of the standard state for the above free energy changes, we must remember that, while mole fractions are strongly recommended composition measures (61 Mil), in practice, both molalities, m, and concentrations, c, are widely used. For dilute aqueous solutions at moderate temperatures the numerical values of m and c are only slightly different. This no longer holds for other solvents. 

In a discussion of permeability it is important to recognize that we deal with operational definitions, since the act of measurement influences the state of the system. In your case, applying an electrical potential gradient and performing electrodialysis alter the distribution of ionophore within the membrane. I wonder whether you have attempted to measure permeability by isotopic tracer techniques In this method the distribution of ionophore would not be influenced. Furthermore, information can be obtained on the question of carriers versus channels or pores. It should not be difficult to determine the extent of possible isotope interaction between tracer species and abundant species in the membrane as discussed by Kedem and Essig [J. Gen. Physiol., 48, 1047 (1965)]. Positive isotope interaction would tend to suggest the presence of channels or pores, negative isotope interaction the presence of carriers. 

This observed behavior of soy proteins complicates the definition of soy protein solubility, the comparison of solubility data, and the interpretation of solubility experiments. Because the thermodynamic criteria are not met, protein solubility becomes an operationally defined quantity that depends upon the experimental methods of measurement. A number of different operational definitions have been used to measure protein solubility. Each has its own advantages and disadvantages, and limited utility. This plurality, though often desirable, makes it difficult to compare experimental results. 

Mitchell used chemiosmotic to describe enzymatic reactions that involve, simultaneously a chemical reaction and a transport process. The operational definition of coupling is shown in Figure 19-18. When isolated mitochondria are suspended in a buffer containing ADP, Pi, and an oxidizable substrate such as succinate, three easily measured processes occur (1) the substrate is oxidized (succinate yields fumarate), (2) 02 is consumed, and (3) ATP is synthesized. Oxygen consumption and ATP synthesis depend on the presence of an oxidizable substrate (succinate in this case) as well as ADP and Pi. 

Conventions Used to Define Nonideality. The equations developed in the preceeding section for the ideal case have really crystallized our operational definition of ideal conditions, since they require (1) that S/AnBm/ /Awt/Bm = 1 at all concentrations, and (2) that the experimental techniques give us a measure of cMweq or its analogs, such as (X i2CiMi)eq... 

Many measurements of pH in sea water have been recorded, usually to within two decimals. If one used an operational definition and could... 

We recognize from our previous experience that pt is a function of the entropy, volume, temperature, or pressure in appropriate combinations and the composition variables. The splitting of into these two terms is not an operational definition, but its justification is obtained from experiment. The quantity pt is the quantity that is measured experimentally, relative to some standard state, whereas the electrical potential of a phase cannot be determined. Neither can the difference between the electrical potentials of two phases alone at the same temperature and pressure generally be measured. Only if the two phases have identical composition can this be done. If the two phases are designated by primes,... 

According to the above definition, a result that is not expressed in SI units can meet the requirements for traceability. Note that in this case each traceability link is established for a stated chemical purpose. It is asserted that such a value is obtained by a measurement applicable to the measurand, in a particular laboratory using a specific procedure, over a time period during which relevant measurement operations are maintained under control with acceptable repeatabilities, and within a limited range of magnitude of the measurand. 

For Tgxp measurements, the sample is scanned along the temperature axis (left part of Fig. 30). In this case, the sample retains the value of the Young modulus typical for the glassy state up to T = T urc. Thereafter the modulus decreases sharply. The difference between T ure and T p is clearly seen and is defined by the operational definition of T. If one defines Tg as the starting point of the E temperature drop, the relation Tcute = Tgxp will hold exactly. From Fig. 30 it is seen that both these temperatures are bound to be connected by the approximate Equation ... 

Selection of a health outcome for study depends in part upon the exposure of interest and methodological considerations such as the ability to define, measure, and validate adverse outcomes, especially if self-reported. Operational definitions for outcomes can be general or specific in nature (e.g., all birth defects versus spina bifida, respectively), and will affect the type of statistical analysis which can be performed and the interpretation of results. Statistical power may be limited if restrictive operational definitions are used for rare outcomes, or if other important covariates cannot be fully addressed. 

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